Implement Encoding on String

[b-n]

This appears to still be hanging over from mruby days (after looking at some of the comments, mruby was UTF-8 only).

Some examples:

• String#force_encoding doesn't change encoding https://github.com/artichoke/artichoke/blob/trunk/artichoke-backend/src/extn/core/string/string.rb#L606-L610
• Encoding class also has many TODOs https://github.com/artichoke/artichoke/blob/trunk/artichoke-backend/src/extn/core/string/string.rb#L84-L150
• Some places present it's actually UTF-8 when the underlying RString might actually be something else: https://github.com/artichoke/artichoke/blob/trunk/artichoke-backend/src/extn/core/string/string.rb#L570-L574

At present, this makes writing tests for things like Time#strftime impossible for non UTF-8 encodings, as mentioned here: #2182 (comment)

[b-n]

@lopopolo - Thoughts on difficulty? Medium?

[lopopolo]

Medium sounds right. We'll probably want to implement the Ruby-side encoding enum for all possible MRI encodings and have a TryFrom to go into spinoso string encoding.

[lopopolo]

Link:

• Implement String::force_encoding and EncodedString::force_encoding #1726

I think this is implemented in spinoso string but needs to be wired up in Artichoke backend.

[lopopolo]

Link:

• Implement Encoding support #183

The approach has significantly changed since this ticket got cut. I think the intent here is full encoding parity with MRI, which is why encoding_rs is mentioned.

Out of scope for this ticket.