parse: add quick path for LITWORD words
parse: add quick path for LITWORD words

name                 old time/op    new time/op    delta
Parse/Whitespace-4     1.57µs ± 1%    1.58µs ± 1%    ~             (p=0.329 n=6+6)
Parse/Comment-4         616ns ± 2%     613ns ± 2%    ~             (p=0.465 n=6+6)
Parse/LongLit-4        2.67µs ± 1%    2.65µs ± 1%    ~             (p=0.167 n=6+6)
Parse/Cmds-4           15.7µs ± 1%    15.4µs ± 1%  -2.25%          (p=0.002 n=6+6)
Parse/Quoted-4         2.33µs ± 1%    2.33µs ± 0%    ~             (p=0.537 n=6+5)
Parse/NestedStmts-4    8.35µs ± 2%    8.26µs ± 1%    ~             (p=0.132 n=6+6)
Parse/Clauses-4        5.67µs ± 1%    5.58µs ± 1%  -1.50%          (p=0.004 n=6+5)
Parse/Binary-4         5.46µs ± 1%    5.38µs ± 1%  -1.46%          (p=0.004 n=5+6)
Parse/Redirect-4       8.04µs ± 1%    8.01µs ± 0%    ~             (p=0.537 n=6+5)
Parse/Arithm-4         5.05µs ± 1%    5.10µs ± 2%    ~             (p=0.240 n=6+6)

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