`std.array.byPair` now returns a `NamedTuple`

$(REF byPair, std, array) now returns a named tuple.

---
import std.array : byPair;
import std.typecons : Tuple;

int[string] dict = ["b": 2, "c": 3];
auto pairs = dict.byPair;
static assert(is(typeof(pairs.front) : Tuple!(string,int)));

// access by index (existing way)
assert(pairs.front[0] == "b");
assert(pairs.front[1] == 2);

// access by name (enabled with this release)
assert(pairs.front.key == "b");
assert(pairs.front.value == 2);
---