[FR] Re-add bcrypt, bin-hex and so on

[duskmoon314]

Previously, we have predefined bcrypt and bin-hex implementation. (At least in v0.13.1, which I have used before)

It seems these implementations were removed when introducing GeneralPurpose. But their alphabets still exist. So please add implementation via the new way.

And thanks for your job. The new method seems convenient for customizing.

[marshallpierce]

The alphabets are still there: https://docs.rs/base64/latest/base64/alphabet/index.html

You can build a GeneralPurpose engine instance with any alphabet you choose via https://docs.rs/base64/latest/base64/engine/general_purpose/struct.GeneralPurpose.html#method.new.