Images dated 050905a and later use slightly different (hopefully
faster in most cases) hash table code.

Some of the changes (how deleted entries are represented) require
parallel changes in the lisp kernel (the GC deletes otherwise
unreferenced entries from :WEAK hash tables.)  Recent kernels should
refuse to load images that're too new or too old for them (a mismatch
is detected very early); it's more important than usual to build a
new kernel after updating from CVS and before trying to bootstrap
from one of these images.

I didn't change the FASL version; FASL files that might contain the
compiled expansion of WITH-HASH-TABLE-ITERATOR probably won't work
correctly (calls to MAPHASH shouldn't be affected.)

Bugs that I've seen (and hopefully fixed) can have weird symptoms,
including infinite loops and stack overflows.  Hopefully, those kinds
of things are -all- fixed ...