pw_tokenizer: Move Base64 functions to class

- Move the Base64 tokenization functions to the tokenizer class, which
  is much cleaner to work with.
- Have AutoUpdatingDetokenizer derive from Detokenizer. This makes
  working with detokenizers simpler.

Change-Id: Ic6bd9354c34f21a9931c83200e7c98e05911b6a2
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47761
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Keir Mierle <keir@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>