Problem:  Every NFA regexp compilation allocates and frees two temporary
          scratch buffers, the postfix list and the fragment stack, even
          though the NFA states themselves are already allocated in a
          single block; this repeats for every pattern, e.g. the hundreds
          of autocmd patterns and syntax items loaded when opening a file.
Solution: Keep the postfix list and the fragment stack allocated between
          compilations, growing them only when a larger one is needed, and
          free them once in free_regexp_stuff().

The backtracking engine already uses this approach: it keeps its "regstack"
and "backpos" scratch buffers across invocations, allocating them only when
not present, and frees them in free_regexp_stuff(). Apply the same reuse to
the NFA "post_start" postfix list and the post2nfa() fragment stack by making
them grow-only statics. Only the scratch memory management changes; the
compiled program and matching are unaffected.

Measured with callgrind (instructions retired), against baseline:
- opening a single C file:                   ~3%
- opening a Vim script file:                 ~2.5%
- cold start opening a file (full runtime):  -3%
- repeated ":runtime! syntax/c.vim" (x40):   -2.5%
- opening 51 real source files in a session: -2%

Signed-off-by: Jules Voisin <julien.voisin@example.com>