Allow multiple hold symbols without spaces

[unlessgames]

Implementing #57

[github-actions]

Benchmark for 8fa6412

Click to view benchmark

Test	Base	PR	%
Cycle/Generate	48.7±0.40µs	52.4±0.57µs	+7.60%
Cycle/Parse	368.9±6.24µs	364.1±4.95µs	-1.30%
Rust Phrase/Clone	442.5±3.65ns	443.5±5.71ns	+0.23%
Rust Phrase/Create	75.0±0.86µs	74.1±0.92µs	-1.20%
Rust Phrase/Run	646.7±5.36µs	650.7±5.45µs	+0.62%
Rust Phrase/Seek	151.6±272.22µs	147.4±277.38µs	-2.77%
Scripted Phrase/Clone	627.8±10.02ns	624.6±7.79ns	-0.51%
Scripted Phrase/Create	950.6±10.72µs	957.7±19.39µs	+0.75%
Scripted Phrase/Run	1669.5±9.46µs	1676.2±24.08µs	+0.40%
Scripted Phrase/Seek	220.4±444.39µs	215.2±434.21µs	-2.36%

[emuell]

I guess it would be possible to only disallow sequences of ___ as name in the parser? So we could also support using "_some_name" as target, but that's likely not worth the extra parsing overhead and complication in the grammar.

[unlessgames]

we could also support using "_some_name" as target

We could try and see how much slower it got but I didn't think it worth the trouble. Also, it might be better to error instead of going with a target silently when the user is really meant to write some _ another and not some _another, not sure how often you'd want to start targets with underscore in general anyway.

[emuell]

Yep. It's good enough as it is now.