mdw-base.lisp: Refactor `with-places' incompatibly.

Now there are two macros.

  * `with-places' is a simple macro which does the hard work of
    capturing places with symbol-macros.

  * `with-places/gensyms' is a hairy nested-backquote macro helper which
    does the job of the old `with-places'.

Because of this split of responsibilities, no callers of these macros
have to mess with environments explicitly, which simplifies
`update-place' and `update-place-after' slightly.