Support UTF-8 label matchers: Add repl for UTF-8 and classic matchers

[grobinson-grafana]

This commit adds a repl for comparing UTF-8 and classic matchers.

./repl
> foo=bar
#   utf-8: {foo="bar"}
# classic: {foo="bar"}
> foo=😃
#   utf-8: {foo="😃"}
# classic: {foo="😃"}
> foo="\xF0\x9F\x99\x82"
#   utf-8: {foo="🙂"}
# classic: {foo="\\xF0\\x9F\\x99\\x82"}