Add let-when-compile macro instead of using pcase-let

Add let-when-compile macro instead of using pcase-let

* lisp/subr.el (let-when-compile): New let-like macro that makes its
bindings known to macros like `eval-when-compile' in the body.

* lisp/emacs-lisp/lisp-mode.el: Change the top-level `pcase-let' to a
  `let-when-compile'. Also comment out the unused lexical var
  `el-kws-re'.

The change greatly improves readability, while providing almost the
same (even shorter) byte code: instead of pre-evaluating 10 variables,
tossing them into a list, and destructuring that list a full screen
page later, the variables are simply bound as they are evaluated,
wrapped individually in `eval-when-compile'.