New integer hash function, rehash bug fix

Version 2.1.0 version introduces two significant changes:

• It replaces the default integer (which was murmur3) to a slightly weaker one (fast-hash) that proved to perform better in benchmarks.
• It fixes a bug that could theoretically cause a crash when the NAME_shrink function prompts a full-table rehash. In testing, I managed to trigger this bug using an (illegal) maximum load factor higher than 1.0.