Add blink-matching-check-function and misc cleanups.

Add blink-matching-check-function and misc cleanups.
* lisp/simple.el (newline): Eliminate optimization.
Use post-self-insert-hook to set hard-newline and things before
running post-self-insert-hook.
(blink-matching-check-mismatch): New function.
(blink-matching-check-function): New variable.
(blink-matching-open): Use them.
Skip back forward over prefix chars skipped by forward-sexp.
Don't check if the parens are backslash escaped.
(blink-paren-post-self-insert-function): Check backslash escaping here.