Use region cache to speedup bidi_find_paragraph_start.

Use region cache to speedup bidi_find_paragraph_start.
* src/buffer.h (struct buffer): New member bidi_paragraph_cache.
Rename cache_long_line_scans to cache_long_scans.
* src/buffer.c (bset_cache_long_line_scans): Rename to
bset_cache_long_scans.
(Fget_buffer_create, Fmake_indirect_buffer, Fkill_buffer)
(Fbuffer_swap_text, init_buffer_once): Take bidi_paragraph_cache
into account.
(syms_of_buffer): Rename cache-long-line-scans to
cache-long-scans.  Adjust docstring.
* src/search.c (newline_cache_on_off):
* src/indent.c (width_run_cache_on_off): Adjust users.
* src/bidi.c (bidi_paragraph_cache_on_off): New function.
(bidi_find_paragraph_start): Use bidi_paragraph_cache if needed.
* src/insdel.c (prepare_to_modify_buffer): Invalidate
bidi_paragraph_cache if enabled.
* doc/lispref/positions.texi (Motion by Screen Lines):
* doc/lispref/display.texi (Truncation): Rename `cache-long-line-scans'
to `cache-long-scans'.