'WRITEABLEP' and macro-expansion-cache

[RWK at MIT-MC (Robert W. Kerns)]

Actually, it is PUREP which MACROMEMO calls.  Even if pure code is
writable, you don't want to put pointers to impure code in it because it
won't be GC protected.  I hope you haven't changed it.

Note that this is the right thing for MACROMEMO to do, independently of
changing DISPLACE.  MACROMEMO wants to do something else rather than just
become a NOP in that case.