Mark hash_bytes_medium as inline never

[hoxxep]

Reduce the inlining cost of FoldHasher::write by marking hash_bytes_medium as #[inline(never)]. This reduces the inlining cost of Hash::hash(str) and means the write_length_prefix and finish methods are more likely to be inlined and efficiently optimise by the compiler.

StrUuid/hashonly-struuid-foldhash-fast                                                                             
                        time:   [3.8693 ns 3.8759 ns 3.8845 ns]
                        change: [-30.347% -30.207% -30.075%] (p = 0.00 < 0.05)
                        Performance has improved.

StrDate/hashonly-strdate-foldhash-fast                                                                             
                        time:   [1.3790 ns 1.3990 ns 1.4221 ns]
                        change: [-74.467% -74.116% -73.656%] (p = 0.00 < 0.05)
                        Performance has improved.

StrEnglishWord/hashonly-strenglishword-foldhash-fast                                                                             
                        time:   [1.4834 ns 1.4860 ns 1.4887 ns]
                        change: [-73.295% -73.243% -73.190%] (p = 0.00 < 0.05)
                        Performance has improved.

StrUrl/hashonly-strurl-foldhash-fast                                                                             
                        time:   [6.2293 ns 6.2748 ns 6.3197 ns]
                        change: [-17.006% -16.186% -15.326%] (p = 0.00 < 0.05)
                        Performance has improved.

Per discussion on #32

[orlp]

Superseded by #35.