Rename comment-depth (etc.) to literal-cache (etc.). Enable it by default

Rename comment-depth (etc.) to literal-cache (etc.).  Enable it by default

* buffer.c:
* buffer.h:
* syntax.c:
* syntax.h:
* insdel.c:
* textprop.c: Rename `comment-depth' to `literal-cache', `comment-depth-hwm'
to `literal-cache-hwm', `comment-cacheing-flag' to `literal-cacheing-flag'.
Rename some functions and internal variables similarly.

* syntax.c (literal-cacheing-flag): Enable by default.