Added `std.conv.bitCast`

This convenience function allows reinterpreting casts to be written in a more
readable way.

---
uint n = 0xDEADBEEF;

// Before
writeln("Bytes of n are: ", *cast(const ubyte[4]*) &n);

// After
writeln("Bytes of n are: ", n.bitCast!(const ubyte[4]));
---