parse: reorder switch order in next()
parse: reorder switch order in next()

Prioritize cases in which tokenizing is the most common.

name                 old time/op    new time/op    delta
Parse/Whitespace-4      707ns ± 0%     710ns ± 2%    ~             (p=0.502 n=6+6)
Parse/Comment-4         401ns ± 0%     408ns ± 1%  +1.69%          (p=0.008 n=5+5)
Parse/LongLit-4        1.60µs ± 1%    1.65µs ± 1%  +3.11%          (p=0.002 n=6+6)
Parse/Cmds-4           12.3µs ± 2%    12.0µs ± 1%  -2.70%          (p=0.004 n=6+5)
Parse/Quoted-4         1.46µs ± 2%    1.45µs ± 1%    ~             (p=0.621 n=6+6)
Parse/NestedStmts-4    6.77µs ± 1%    6.67µs ± 1%  -1.45%          (p=0.002 n=6+6)
Parse/Clauses-4        4.17µs ± 1%    4.06µs ± 1%  -2.67%          (p=0.002 n=6+6)
Parse/Binary-4         4.33µs ± 1%    4.31µs ± 0%  -0.56%          (p=0.030 n=6+5)
Parse/Redirect-4       4.32µs ± 1%    4.23µs ± 0%  -2.15%          (p=0.004 n=6+5)
Parse/Arithm-4         4.29µs ± 1%    4.26µs ± 1%    ~             (p=0.093 n=6+6)

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