2.1.0: Fix encoding inconsistencies & accept String-like inputs with RE2::Set

As RE2 only supports UTF-8 and ISO-8859-1 encodings, fix an inconsistency when using string patterns with RE2.Replace and RE2.GlobalReplace where the result would match the encoding of the pattern rather than UTF-8 which is what RE2 will use. This behaviour and limitation is now documented on all APIs that produce string results.

e.g.

# With re2 2.0.0
RE2.Replace("Hello World", "Hello".encode("Windows-1250"), "Hi").encoding
# => #<Encoding:Windows-1250>

# With re2 2.1.0
RE2.Replace("Hello World", "Hello".encode("Windows-1250"), "Hi").encoding
# => #<Encoding:UTF-8>

Other changes

• The RE2::Set API now accepts anything that can be coerced to a string where previously only strings were permitted, e.g. RE2::Set#add, RE2::Set#match.
• Added the licences of all vendored dependencies: RE2 and Abseil.
• Document the behaviour of RE2::Regexp#match when given a pattern with no capturing groups: it will return true or false whether there was a match or not rather than a RE2::MatchData instance.

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