Code cleanup for long line optimizations

Code cleanup for long line optimizations

This commit does not change any code, it merely renames functions
and clarifies the documentation, to make the code hopefully easier
to grasp.

* src/dispextern.h (struct it): Rename the 'narrowed_begv',
'narrowed_zv', 'locked_narrowing_begv', 'locked_narrowing_zv' to
'medium_narrowing_begv', 'medium_narrowing_zv',
'large_narrowing_begv', 'large_narrowing_zv'.  Clarify the
comments.
Update the prototypes of the functions renamed in xdisp.c.

* src/lisp.h: Update the prototypes of the functions renamed in
editfns.c.  Remove the prototype of
'safe_run_hooks_maybe_narrowed', which is used only in keyboard.c.

* src/xdisp.c
(get_small_narrowing_begv): Renamed from
'get_closer_narrowed_begv'.
(get_medium_narrowing_begv): Renamed from 'get_narrowed_begv'.
(get_medium_narrowing_zv): Renamed from 'get_narrowed_zv'.
(get_large_narrowing_begv): Renamed from 'get_locked_narrowing_begv'.
(get_large_narrowing_zv): Renamed from 'get_locked_narrowing_zv'.
(SET_WITH_NARROWED_BEGV): Use the new field names.
(handle_fontified_prop): Use the new function and new field names.
(back_to_previous_line_start): Use the new field name.
(back_to_previous_visible_line_start): Use the new field name.
(reseat): Use the new function and new field names.
(get_visually_first_element): Use the new field name.
(move_it_vertically_backward): Use the new function name.
(redisplay_internal): Use the new function name.
Also add a large comment to explain how Emacs deals with long lines.

* src/keyboard.c:
(safe_run_hooks_maybe_narrowed): Use the new function names from
xdisp.c and editfns.c.  Make the function static, and add a
prototype.

* src/editfns.c:
(labeled_restrictions): Renamed from 'narrowing_locks'.
(labeled_restrictions_add): Renamed from 'narrowing_locks_add'.
(labeled_restrictions_remove): Renamed from
'narrowing_locks_remove'.
(labeled_restrictions_get_bound): Renamed from
'narrowing_lock_get_bound'.
(labeled_restrictions_peek_label): Renamed from
'narrowing_lock_peek_tag'.
(labeled_restrictions_push): Renamed from 'narrowing_lock_push'.
(labeled_restrictions_pop): Renamed from 'narrowing_lock_pop'.
(unwind_reset_outermost_restriction): Renamed from
'unwind_reset_outermost_narrowing'.
(reset_outermost_restrictions): Renamed from
'reset_outermost_narrowings'.
(labeled_restrictions_save): Renamed from 'narrowing_locks_save'.
(labeled_restrictions_restore): Renamed from
'narrowing_locks_restore'.
(unwind_labeled_narrow_to_region): Renamed from
'unwind_narrow_to_region_locked'.
(labeled_narrow_to_region): Renamed from
'narrow_to_region_locked'.
(Finternal__label_restriction): Renamed from
'Finternal__lock_narrowing'.
(Finternal__unlabel_restriction): Renamed from
'Finternal__unlock_narrowing'.
(Fwiden): Use the new function names.
(Fnarrow_to_region): Use the new function names.
(save_restriction_save): Use the new function names.
(syms_of_editfns): Use the new function names.
<outermost-restriction>: Renamed from 'outermost-narrowing'.

* lisp/subr.el (internal--with-restriction): Use the new internal
function name.
(internal--without-restriction): Use the new internal function
name.

* src/composite.c (composition_compute_stop_pos):
(find_automatic_composition): Use the new function name.

* doc/lispref/positions.texi (Narrowing): Add index entry.