Make equality checks faster and fix hash

[oxinabox]

solves #15

With this PR:

julia> @btime $(ShortString15("abc")) == "abc";
  10.935 ns (0 allocations: 0 bytes)

julia> @btime $(ShortString15("abc")) == "abd";
  11.152 ns (0 allocations: 0 bytes)

Without this PR

julia> @btime $(ShortString15("abc")) == "abc";
  79.701 ns (3 allocations: 224 bytes)

julia> @btime $(ShortString15("abc")) == "abd";
  80.259 ns (3 allocations: 224 bytes)

This also correct the hash closing #21

[oxinabox]

once we have this, it would be useful to tag a release as I want to use this for something

[xiaodaigh]

Thanks!

[oxinabox]

..