search.c (re--describe-compiled): New function (bug#66261)

search.c (re--describe-compiled): New function (bug#66261)

This provides a fairly primitive but handy way to see what
a regexp compiles to without having to enable REGEX_EMACS_DEBUG
and wade through tons of stderr output.

* doc/lispref/searching.texi (Regexp Problems): Mention
`re--describe-compiled`.

* src/regex-emacs.c (debug_putchar, print_fastmap)
(print_partial_compiled_pattern, print_compiled_pattern): Add `dest`
argument, and compile also when `ENABLE_CHECKING` is set.
(DEBUG_PRINT_COMPILED_PATTERN, print_double_string, regex_compile):
Adjust to additional argument.

* src/regex-emacs.h (print_compiled_pattern): Declare.

* src/search.c (Fre__describe_compiled): New function.
(syms_of_search): Defsubr it.