Update Encoding::Converter

[sampersand]

This updates Encoding::Converter.

More specifically, it:

• Encoding::Converter::encoding: Removed (lives in builtin.rbs)
• Moved constants within Encoding::Converter
• {search_convpath,initialize}: Added flags variant, split apart newlines vs explicit newline arguments
• ==: RHS is now untyped
• {convert,insert_output,putback} now accepts implicit args
• convpath no longer returns conversion_path (It's not [encoding,encoding] but [Encoding,Encoding])
• last_error: Merged return types
• primitive_convert: Now takes implicit int and strings, also added flags variant
• replacement= now takes boolish
• Added tests for the above

TODO: finalize tests