Fix issue 21926 - Allow leadings zeros in std.conv.octal

First of all, the DDoc currently says that:

"Leading zero is allowed, but not required."

The current implementation doesn't respect that.

D lexer allows leading zero(s) also.

And when considering octal from string, there is no possibility
of confusion, as the comments in the code claim. Disallowing
leading zeros for octal conversion from string, does not help
with anything actually.

So lets allow leading zeros.

Allowing leading zeros, does help when implementing various
APIs (i.e. glibc, Linux kernel), where a lot of octal constant,
are defined with multiple leading zeros, for readability and
alignment, in C headers. Having to manually or automatically
special case these when porting such API definitions, is
counter-productive.

Example from a Linux kernel (`include/uapi/linux/stat.h`):

```c
#define S_IFMT  00170000
#define S_IFSOCK 0140000
#define S_IFLNK	 0120000
#define S_IFREG  0100000
#define S_IFBLK  0060000
#define S_IFDIR  0040000
#define S_IFCHR  0020000
#define S_IFIFO  0010000
#define S_ISUID  0004000
#define S_ISGID  0002000
#define S_ISVTX  0001000
```

With this patch, now it is trivial and easier to convert these to D:

```d
...
enum S_ISVTX = octal!"0001000";
```

while being close to original. That helps with readability,
and long term maintenance.

In fact the run-time version provided by `parse!(int)(string, 8)`
also supports leading zeros already. So this makes `octal`
more consistent `parse`.

std/conv.d

@@ -2884,6 +2884,7 @@ do
 
     assert(parse!int(s = "0011001101101", 2) == 0b0011001101101);
     assert(parse!int(s = "765", 8) == octal!765);
+    assert(parse!int(s = "000135", 8) == octal!"135");
     assert(parse!int(s = "fCDe", 16) == 0xfcde);
 
     // https://issues.dlang.org/show_bug.cgi?id=6609
@@ -4841,12 +4842,14 @@ if (is(typeof(decimalInteger)) && isIntegral!(typeof(decimalInteger)))
 ///
 @safe unittest
 {
-    // same as 0177
+    // Same as 0177
-    auto x = octal!177;
+    auto a = octal!177;
     // octal is a compile-time device
-    enum y = octal!160;
+    enum b = octal!160;
     // Create an unsigned octal
-    auto z = octal!"1_000_000u";
+    auto c = octal!"1_000_000u";
+    // Leading zeros are allowed when converting from a string
+    auto d = octal!"0001_200_000";
 }
 
 /*
@@ -4877,6 +4880,9 @@ private T octal(T)(const string num)
 {
     int a = octal!int("10");
     assert(a == 8);
+
+    int b = octal!int("000137");
+    assert(b == 95);
 }
 
 /*
@@ -4896,12 +4902,41 @@ private template octalFitsInInt(string octalNum)
 private string strippedOctalLiteral(string original)
 {
     string stripped = "";
+    bool leading_zeros = true;
     foreach (c; original)
-        if (c >= '0' && c <= '7')
+    {
+        if (!('0' <= c && c <= '7'))
+            continue;
+        if (c == '0')
+        {
+            if (leading_zeros)
+                continue;
+        }
+        else
+        {
+            leading_zeros = false;
+        }
         stripped ~= c;
+    }
+    if (stripped.length == 0)
+    {
+        assert(leading_zeros);
+        return "0";
+    }
     return stripped;
 }
 
+@safe unittest
+{
+    static assert(strippedOctalLiteral("7") == "7");
+    static assert(strippedOctalLiteral("123") == "123");
+    static assert(strippedOctalLiteral("00123") == "123");
+    static assert(strippedOctalLiteral("01230") == "1230");
+    static assert(strippedOctalLiteral("0") == "0");
+    static assert(strippedOctalLiteral("00_000") == "0");
+    static assert(strippedOctalLiteral("000_000_12_300") == "12300");
+}
+
 private template literalIsLong(string num)
 {
     static if (num.length > 1)
@@ -4925,8 +4960,8 @@ private template literalIsUnsigned(string num)
 /*
 Returns if the given string is a correctly formatted octal literal.
 
-The format is specified in spec/lex.html. The leading zero is allowed, but
+The format is specified in spec/lex.html. The leading zeros are allowed,
-not required.
+but not required.
  */
 @safe pure nothrow @nogc
 private bool isOctalLiteral(const string num)
@@ -4934,21 +4969,19 @@ private bool isOctalLiteral(const string num)
     if (num.length == 0)
         return false;
 
-    // Must start with a number. To avoid confusion, literals that
+    // Must start with a digit.
-    // start with a '0' are not allowed
-    if (num[0] == '0' && num.length > 1)
-        return false;
     if (num[0] < '0' || num[0] > '7')
         return false;
 
     foreach (i, c; num)
     {
-        if ((c < '0' || c > '7') && c != '_') // not a legal character
+        if (('0' <= c && c <= '7') || c == '_')  // a legal character
-        {
+            continue;
+
         if (i < num.length - 2)
             return false;
-            else   // gotta check for those suffixes
+
-            {
+        // gotta check for those suffixes
         if (c != 'U' && c != 'u' && c != 'L')
             return false;
         if (i != num.length - 1)
@@ -4962,8 +4995,6 @@ private bool isOctalLiteral(const string num)
                 return false; // repeats are disallowed
         }
     }
-        }
-    }
 
     return true;
 }
@@ -4981,6 +5012,9 @@ private bool isOctalLiteral(const string num)
     static assert(octal!"7" == 7);
     static assert(octal!"10" == 8);
     static assert(octal!"666" == 438);
+    static assert(octal!"0004001" == 2049);
+    static assert(octal!"00" == 0);
+    static assert(octal!"0_0" == 0);
 
     static assert(octal!45 == 37);
     static assert(octal!0 == 0);
@@ -4989,6 +5023,7 @@ private bool isOctalLiteral(const string num)
     static assert(octal!666 == 438);
 
     static assert(octal!"66_6" == 438);
+    static assert(octal!"0_0_66_6" == 438);
 
     static assert(octal!2520046213 == 356535435);
     static assert(octal!"2520046213" == 356535435);