Use a more efficient method of duplicate detection

See, interview questions *are* useful in the real world!
I can spell!
Add some more detail to PHF_STATS output
Update for Vec changes
Clean up entries iterator a bit
Redo entry storage and iteration

We always have a 1.0 load factor, so there's really no reason to wrap
with an Option. This also simplifies the iterator and gives a correct
size_hint.
Update to new inner attr syntax
Implement Show for PhfMap
Remove a weird match
Some random cleanup
Update for visibility overhaul
Update README
Style cleanup
Update for AST change
Minor clarification in iterator docs
Moar cleanup
Rust version changes for Servo compatibility -- do not merge upstream