Fix mutation of string literals

[sheldonh]

Ruby 3.4.5:

bin/rake test
...
./home/sheldonh/git/json-stream/lib/json/stream/parser.rb:187: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
/home/sheldonh/git/json-stream/lib/json/stream/parser.rb:555: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
/home/sheldonh/git/json-stream/lib/json/stream/parser.rb:187: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
/home/sheldonh/git/json-stream/lib/json/stream/parser.rb:187: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
.

Finished in 0.025354s, 5916.2034 runs/s, 10294.1939 assertions/s.

150 runs, 261 assertions, 0 failures, 0 errors, 0 skips

[sheldonh]

To really see what's going on:

~/git/json-stream (master =)$ RUBYOPT="--enable=frozen-string-literal" bin/rake test
Run options: --seed 17194

# Running:

EEEEEEEEEEE..EEEE.EEEEEE.EEEEEEEEEEE.EEEEEEE..E.EEE.E.E.EEEE.EFE.EE...E......FFFFFEEEEFFFEE.EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE.EEEE..EE.EE.EEEEEEEEEE.EE

Finished in 0.017250s, 8695.4697 runs/s, 3652.0973 assertions/s.

  1) Error:
JSON::Stream::Parser::parsing string tokens::parsing unicode escapes with surrogate pairs#test_0001_rejects missing second pair:
FrozenError: can't modify frozen String: ""
    lib/json/stream/parser.rb:217:in 'block in JSON::Stream::Parser#<<'
    lib/json/stream/parser.rb:155:in 'String#each_char'
    lib/json/stream/parser.rb:155:in 'JSON::Stream::Parser#<<'
    spec/parser_spec.rb:880:in 'block in events'
    spec/parser_spec.rb:880:in 'String#each_byte'
    spec/parser_spec.rb:880:in 'events'
    spec/parser_spec.rb:563:in 'block (4 levels) in <top (required)>'
...

test.log