Coerce symbols internal fstrings in UTF8 rather than ASCII

[casperisfine]

Ref: https://bugs.ruby-lang.org/issues/15940

It's not uncommon for symbols to have literal string counterparts, e.g.

class User
  attr_accessor :name

  def as_json
    { 'name' => name }
  end
end

Since the default source encoding is UTF-8, and that symbols coerce their
internal fstring to ASCII when possible, the above snippet will actually keep
two instances of "name" in the fstring registry. One in ASCII, the other
in UTF-8.

Considering that UTF-8 is a strict superset of ASCII, storing the symbols
fstrings as UTF-8 instead makes no real difference, but allows in most cases
to reuse the equivalent string literals.

The only notable behavioral change is Symbol#to_s.

Previously :name.to_s.encoding would be #<Encoding:US-ASCII>.
After this patch it's #<Encoding:UTF-8>. I can't foresee any significant
compatibility impact of this change on existing code.

There are several ruby specs asserting this behavior. If this specification is impossible to change, then we could consider changing the encoding of the String returned by Symbol#to_s, e.g )ruby pseudo code:

def to_s
  str = fstr.dup
  str.force_encoding(Encoding::ASCII) if str.ascii_only?
  str
end

[casperisfine]

@eregon CI is now green, please let me know if I made mistake in the specs.

[k0kubun]

Now master branch has a required check "check_branch", but I couldn't update your branch to include it. Could you rebase this from master to run it?

[casperisfine]

I rebased and the check_branch status was properly triggered.