[COMMIT] If STRING is constant, call regexp-quote at compile time.

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > A small thing, I admit, but it’s nice to endorse this practice. 

Two comments on the poor documentation of this feature:

If you want to endorse it, document it, please.  Definitely requires a
comment in the Lispref, maybe in the docstring.  What you've done so
far is more along the lines of "idiots will do this, so make sure
there's no run-time cost."

I wasted five minutes reviewing this before I realized it was a
no-brainer.  A subject like "Optimize regexp-quote: apply to constant
strings at compile time" would have made it much clearer what is being
done, and why.