Reconsider match_byte macro if plain match becomes optimized enough

[SimonSapin]

@nox pointed out (#112 (comment)) that rust-lang/rust#39456 brings the same performance to plain match expressions.

[SimonSapin]

1.17.0 is stable, but the tests::unquoted_url benchmark is ~2x slower when compiled with --features dummy_match_byte. CC @nox, @emilio

[nox]

Sad!

[SimonSapin]

Still a ~2x difference in 1.71