parse_test: new benchmarks
parse_test: new benchmarks

* Be more consistent about sizes and repetitions
* Collapse clauses
* Add arithmetic sample

name                 time/op
Parse/Empty-4        1.11µs ± 5%
Parse/Whitespace-4   7.32µs ± 1%
Parse/Comment-4      3.68µs ± 4%
Parse/LongLit-4      17.5µs ± 8%
Parse/Cmds-4         29.8µs ± 3%
Parse/SglQuoted-4    6.08µs ± 3%
Parse/DblQuoted-4    8.28µs ± 1%
Parse/NestedStmts-4  23.2µs ± 3%
Parse/Clauses-4      18.4µs ± 3%
Parse/CmdSubst-4     20.9µs ± 2%
Parse/Binary-4       13.6µs ± 3%
Parse/Redirect-4     14.2µs ± 4%
Parse/Arithm-4       25.0µs ± 2%

mvdan/sh,mvdan/sh,mvdan/sh,mvdan/sh