issue MACRO-ENVIRONMENT-EXTENT, version 2

[Glenn S. Burke <gsb@ALDERAAN.SCRC.Symbolics.COM>]

I believe in dynamic extent for macro environments.  The efficiency
penalty of doing otherwise increases as more functionality becomes
attached to macro environments.

I have serious doubts that adding a copier for macro environments would
help, because of the quantity of data which might need to be copied:
this could conceivably be a compiler's entire model of a compilation
destination (type proclamations, defmacro definitions, etc).

Most if not all of the examples i've seen which purport to require
indefinite extent for macroexpansion environments would be better done
(easier to read and understand, easier to implement efficiently) by
special constructs designed for the purpose.