Add internal function to enter a labeled restriction

Add internal function to enter a labeled restriction

* src/editfns.c (Finternal__labeled_narrow_to_region): New
function.  A specific function is necessary to avoid unnecessary
slowdowns when 'narrow-to-region'/'widen' are called in a loop.
(Fnarrow_to_region): Remove the call to Fset, which has been moved
into Finternal__labeled_narrow_to_region.
(labeled_narrow_to_region): Use the new function.
(syms_of_editfns): Add the symbol of the new function.

* lisp/subr.el (internal--with-restriction): Use the new function.