Compile time assert

[davemilter]

@sfackler

You can abuse transmute to do this at compile time if you want:

fn _size_check() { unsafe { mem::transmute::<usize, [u8; 4]>(0); } }

Yes, I know about this trick, there is also static_assertions crate that provides nice wrapper around this or similar trick.

But it would be great to have it inside std or compiler.