Release 3.0.0

[ia0]

Required tasks:

• [x] Use static dispatch instead of dynamic dispatch
• [x] Use static instead of const for pre-defined encodings
• [x] Make Encoding implementation private
• [x] Add _uninit() version to support MaybeUninit outputs
• [ ] Introduce explicit traits to avoid users seeing True and False (also useful to avoid parameter ordering issues)
• [ ] Restrict symbols to printable/graphical ASCII?
• [ ] Add v3-preview support to data-encoding-macro
• [ ] Improve documentation of v3-preview

Optional tasks:

• [ ] Identify and fix performance regression with "wrap" functions
• [ ] Add SIMD support (see #95)
• [ ] Consider whether a "short input" engine for base58, nix-base32, etc would be worth it (see #110)

After release:

• [ ] Update OSS-Fuzz to remove the v3-preview mention.