Use unicode \alpha in front-page example.

I tested this using BrowserStack on browsers going back to IE 6 on Windows XP.
It worked everywhere, even though lots of other parts of the (old) web pages didn't.
Use "explain" on the explanation button for the short front-page examples.
Always use a monospace font, even on old browsers.
Improve letrec-values type-checking

Split into two passes in order to be able to find
more cases where the RHS doesn't refer to the bindings
in the letrec.

Closes PR 13124
[fixup] split up the implementation into several pieces

This follows the PR feedback from Eric