parse: use a sync.Pool of bufio.Readers
parse: use a sync.Pool of bufio.Readers

This avoids the alloc of the bufio.Reader object, but most importantly
of the buffer itself.

name                 old time/op    new time/op    delta
Parse/Whitespace-4     1.58µs ± 2%    1.44µs ± 2%   -8.62%  (p=0.002 n=6+6)
Parse/Comment-4         614ns ± 2%     454ns ± 1%  -26.01%  (p=0.004 n=6+5)
Parse/LongLit-4        2.66µs ± 1%    2.51µs ± 2%   -5.65%  (p=0.002 n=6+6)
Parse/Cmds-4           15.3µs ± 2%    15.1µs ± 1%     ~     (p=0.093 n=6+6)
Parse/Quoted-4         2.33µs ± 2%    2.20µs ± 2%   -5.93%  (p=0.002 n=6+6)
Parse/NestedStmts-4    8.20µs ± 1%    8.09µs ± 1%   -1.44%  (p=0.004 n=6+6)
Parse/Clauses-4        5.57µs ± 1%    5.44µs ± 1%   -2.29%  (p=0.004 n=6+5)
Parse/Binary-4         5.35µs ± 1%    5.25µs ± 1%   -1.89%  (p=0.002 n=6+6)
Parse/Redirect-4       7.98µs ± 1%    7.95µs ± 2%     ~     (p=0.699 n=6+6)
Parse/Arithm-4         5.04µs ± 0%    4.93µs ± 1%   -2.19%  (p=0.002 n=6+6)

name                 old alloc/op   new alloc/op   delta
Parse/Whitespace-4       416B ± 0%       64B ± 0%  -84.62%  (p=0.002 n=6+6)
Parse/Comment-4          648B ± 0%      296B ± 0%  -54.32%  (p=0.002 n=6+6)
Parse/LongLit-4        1.31kB ± 0%    0.96kB ± 0%  -26.83%  (p=0.002 n=6+6)
Parse/Cmds-4           6.26kB ± 0%    5.91kB ± 0%   -5.60%  (p=0.002 n=6+6)
Parse/Quoted-4         1.30kB ± 0%    0.95kB ± 0%  -26.99%  (p=0.002 n=6+6)
Parse/NestedStmts-4    3.70kB ± 0%    3.34kB ± 0%   -9.52%  (p=0.002 n=6+6)
Parse/Clauses-4        2.29kB ± 0%    1.94kB ± 0%  -15.38%  (p=0.002 n=6+6)
Parse/Binary-4         3.22kB ± 0%    2.87kB ± 0%  -10.92%  (p=0.002 n=6+6)
Parse/Redirect-4       3.47kB ± 0%    3.12kB ± 0%  -10.11%  (p=0.002 n=6+6)
Parse/Arithm-4         2.02kB ± 0%    1.66kB ± 0%  -17.46%  (p=0.002 n=6+6)

name                 old allocs/op  new allocs/op  delta
Parse/Whitespace-4       3.00 ± 0%      1.00 ± 0%  -66.67%  (p=0.002 n=6+6)
Parse/Comment-4          7.00 ± 0%      5.00 ± 0%  -28.57%  (p=0.002 n=6+6)
Parse/LongLit-4          16.0 ± 0%      14.0 ± 0%  -12.50%  (p=0.002 n=6+6)
Parse/Cmds-4              218 ± 0%       216 ± 0%   -0.92%  (p=0.002 n=6+6)
Parse/Quoted-4           22.0 ± 0%      20.0 ± 0%   -9.09%  (p=0.002 n=6+6)
Parse/NestedStmts-4       101 ± 0%        99 ± 0%   -1.98%  (p=0.002 n=6+6)
Parse/Clauses-4          71.0 ± 0%      69.0 ± 0%   -2.82%  (p=0.002 n=6+6)
Parse/Binary-4           74.0 ± 0%      72.0 ± 0%   -2.70%  (p=0.002 n=6+6)
Parse/Redirect-4         88.0 ± 0%      86.0 ± 0%   -2.27%  (p=0.002 n=6+6)
Parse/Arithm-4           61.0 ± 0%      59.0 ± 0%   -3.28%  (p=0.002 n=6+6)

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