Support non-UTF-8 encodings in :unicode.characters_to_binary/3

[mward-sudo]

As a follow-up to the :unicode module refactor, complete the TODO comments in the characters_to_binary/3 function, adding support for non UTF-8 encoding similar to :unicode.characters_to_list/1.

[bartblast]

Hey @mward-sudo , I've left a detailed comment on #700 that also applies to this PR - could you check it out? Thanks!

[bartblast]

Reopening - this is still planned. The approach will follow the incremental strategy discussed in #700.

[mward-sudo]

I'll turn this in to a tracker for individual implementations.