Add a feature for each bit-width

[ia0]

This is a breaking change for those who use default-features = false. The fix is to add back the all feature.

Fixes #97

[ia0]

Maybe this should be a major change, in which case it will be possible to also ship the following breaking changes from #72 before releasing:

• Making the bases static instead of const.
• Making the Encoding definition private.
• Bumping MSRV.
• If there's time, provide &mut [MaybeUninit<u8>] versions.

[ia0]

I think it should be a major change. Let's try to see what it would take to expose the polymorphic functions. Maybe the types can be hidden with type aliases:

pub type Base64 = Encoding<Msb, Bit6, Pad, NoWrap>;

[ia0]

Closing this in favor of using dyn traits.