Steel Bank Common Lisp

Attached a patch (based on one by Richard Kreuter) that makes PROGV
and SET check that the bindings/assignments don't violate constantness
or declared type of the special variable.

Also attached a patch that fixes the alien-stack leakage -- using a
more efficient approach suggested by Alistair Bridgewater on x86 and
x86-64. (This one actually outperforms the current one if a single
WITH-ALIEN form establishes 4 or more local bindings.)

Since neither of these is a regression, I am not committing the fixes
during the freeze.

Cheers,

 -- Nikodemus Siivola