Full support for tagged enums

I'll be interested in PRs for this issue if `endian_codec` gets abandoned.

Sometimes messages coming from the wire may be different, tagged with a number. It'd be nice to have a full support for enums with specified tag type. It should be done in a way that allows the decoder to not know the size of the message before the tag is received. This may need async code.

I originally intended to implement this feature, but didn't due to lack of time.

The issue names no files, tests, or entry points. First map the current enum and decoder implementation, then review how endian_codec handles tagged enums. Done means supporting enums with a specified tag type while allowing decoding before the message size is known, with async behavior if required.

Written by the indexing model from the issue text.