Expose some protected members

[ugumba]

• Make internal algorithms protected so they can be used from derived classes.
• Move LookupTable inside Base32Encoding to make this work without making it public - happy to make it public if you want.
• Use Alphabet directly from GetOrCreateLookupTable() (optimization).

The story: I need to encode from/decode to a fixed number of bytes in a struct, not a byte[].
The cleanest way to make use of your excellent code was to interface with the 4 methods taking "byte *" as unencoded data.

Alternatively, if ToBase32Unsafe() and ToBytesUnsafe() took "byte *", I could interface with those 2 methods instead - but I felt that change was a bit more invasive, and would be less performant (in my code, lengths/groupCounts/remainders can be const).

Cheers!

[ugumba]

By the way, if you'd rather not merge this, I respect that, but would appreciate to know early - so I can plan on using the fork.
Sticking with your nuget package would be the most convenient path for me, obviously!