Optimization opportunities

[joshlf]

This issue tracks potential opportunities for optimization.

• Per @djkoloski in Allow dereferencing an Unalign, add trait impls #126 (comment), consider modifying the following:
  

  zerocopy/src/lib.rs

  Line 2048 in 8201cbf

  let remainder = t.addr() % mem::align_of::<U>();

  NB: This can just be t.addr() & (mem::align_of::<U>() - 1) == 0. If you want to micro-optimize (you might not) then that might give slightly better codegen since we know mem::align_of always returns a power of two.

• Prevent panics statically #202
• Use "likely" hint to compiler in situations like this one