Implement FromBase64 for &[u8].

The algorithm was already based on bytes internally.

Also use byte literals instead of casting u8 to char for matching.
auto merge of #15810 : SimonSapin/rust/base64-from-bytes, r=alexcrichton

The algorithm was already based on bytes internally.

Also use byte literals instead of casting u8 to char for matching.

[breaking-change] The `InvalidBase64Character` variant of the `FromBase64Error` enum was renamed to `InvalidBase64Byte`, and contains a `u8` instead of a `char`.

r? @alexcrichton