parse: make all byte sets funcs instead
parse: make all byte sets funcs instead

Seems to help greatly.

ok      github.com/mvdan/sh     12.169s
name                 old time/op    new time/op    delta
Parse/Empty-4          1.09µs ± 4%    1.09µs ± 3%     ~           (p=0.869 n=10+10)
Parse/Whitespace-4     7.47µs ± 2%    7.69µs ± 2%   +2.92%        (p=0.000 n=10+10)
Parse/Comment-4        3.64µs ± 3%    3.58µs ± 3%   -1.85%         (p=0.010 n=10+9)
Parse/LongLit-4        16.8µs ± 3%    13.3µs ± 2%  -20.89%        (p=0.000 n=10+10)
Parse/Cmds-4           25.1µs ± 3%    23.2µs ± 1%   -7.64%         (p=0.000 n=10+9)
Parse/SglQuoted-4      4.94µs ± 1%    4.88µs ± 1%   -1.26%        (p=0.001 n=10+10)
Parse/DblQuoted-4      7.01µs ± 2%    6.90µs ± 1%   -1.56%        (p=0.004 n=10+10)
Parse/NestedStmts-4    19.1µs ± 1%    17.8µs ± 1%   -6.94%         (p=0.000 n=10+9)
Parse/Clauses-4        16.5µs ± 2%    14.9µs ± 2%   -9.29%        (p=0.000 n=10+10)
Parse/CmdSubst-4       16.0µs ± 2%    15.3µs ± 1%   -4.30%        (p=0.000 n=10+10)
Parse/Binary-4         11.9µs ± 2%    11.2µs ± 1%   -5.47%         (p=0.000 n=10+8)
Parse/Redirect-4       12.9µs ± 1%    11.9µs ± 1%   -7.74%        (p=0.000 n=10+10)
Parse/Arithm-4         19.3µs ± 2%    17.0µs ± 2%  -12.03%        (p=0.000 n=10+10)

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