gogrep: don't copy capture slice during backtracking

[quasilyte]

Since we use slices now, we can avoid copying when backtracking.

Instead of copying a slice to a restart record, we store a slice
handle of the current length. When backtracking is involved, we
discard all excessive captures by the fact that elements past
length would be forgotten.

This makes gogrep more efficient for the patterns that backtrack.

name                           old time/op    new time/op    delta
Match/captureBacktrackLeft-8     4.67µs ± 3%    3.20µs ± 3%  -31.63%  (p=0.008 n=5+5)
Match/captureBacktrackRight-8    2.45µs ± 1%    1.85µs ± 2%  -24.15%  (p=0.008 n=5+5)

name                           old alloc/op   new alloc/op   delta
Match/captureBacktrackLeft-8       848B ± 0%      368B ± 0%  -56.60%  (p=0.008 n=5+5)
Match/captureBacktrackRight-8      432B ± 0%      208B ± 0%  -51.85%  (p=0.008 n=5+5)

name                           old allocs/op  new allocs/op  delta
Match/captureBacktrackLeft-8       21.0 ± 0%      11.0 ± 0%  -47.62%  (p=0.008 n=5+5)
Match/captureBacktrackRight-8      12.0 ± 0%       6.0 ± 0%  -50.00%  (p=0.008 n=5+5)