[HEADS UP} dynamic-extent merged

I've merged the dynamic-extent stuff into HEAD, so that I can tick
that off.  Please use boot16.lisp for bootstrapping.

Raymond, I've left all the Sparc files unchanged, as there are
differences between the branch and HEAD, and I don't know what should
go in.  *TRUST-DYNAMIC-EXTENT-DECLARATIONS* is set to nil for non-x86.