Add more comprehensive benchmarks
Consolidate packing / unpacking code

Mostly for safety and maintainability.  Between repr.rs and the representation
tests in atom/mod.rs, we now have

Thanks to the magic of LLVM's inlining and SROA, this code is faster on some
benchmarks and not significantly slower on any.

Also added debug_assert!s and changed the existing non-test assert!s.  I
believe they're all safe to omit in release builds.
Use #[unsafe_no_drop_flag] so that Atom can be only 64 bits

Drop flags are going away, hopefully, but in the mean time here's a simple fix.
Use lazy-static for the global hash table
Use XXHash instead of SipHash

This is an approximately 8% speedup on interning a 56-byte string.