Switch from string keys to ReadOnlyMemory<byte>

[NickStrupat]

This way, any byte-representation can be used as a key.

[odinmillion]

Amazing PR! It is a good proposal to generalize library.

I am a little bit confused on this line: https://github.com/odinmillion/Phf.Net/pull/1/files#diff-c0abc29d441dd7573197a5233567ba9bR196
Maybe we have to pack bytes denser: 4 bytes per int32 instead of the 2 bytes.

[NickStrupat]

Ah yes, I missed that. Should the 16 be 8? In both places (https://github.com/odinmillion/Phf.Net/pull/1/files#diff-c0abc29d441dd7573197a5233567ba9bR207)

[odinmillion]

It should be a little trickier) In case of C# strings we map each char to 2 bytes. Therefore we have only 2 possibilities: zero bytes remainder (str.Length % 2 == 0) and 2 bytes reminder (str.Length % 2 == 1). After switching to the bytes we have 4 possibilities for remainder length: 0, 1, 2, 3. Therefore we have to handle all that cases. Later I will try to implement this.