ENCODER: Refactor provider implementations, and some cleanup

ENCODER: Refactor provider implementations, and some cleanup

The encoder implementations were implemented by unnecessarily copying
code into numerous topical source files, making them hard to maintain.
This changes merges all those into two source files, one that encodes
into DER and PEM, the other to text.

Diverse small cleanups are included.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12803)