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Use Encoding::UTF-8 instead of "UTF-8".
Revert "Use Encoding::UTF-8 instead of "UTF-8"."

This reverts commit c326cd9201cabf9d38c1a69882549f3c040eb635.

This resulted "code converter not found (UTF-8 to UTF-8)
(Encoding::ConverterNotFoundError)" in yy_char_range() in IO#getc().
Enforce UTF-8 encoding where needed only.

Try to replace "UTF-8" with Encoding::UTF-8. The try has failed.
Regenerate parser generator.
Make "LEFT ARROW" Unicode symbol equivalent to "<-" and "=".