Greatly simplify configuration choices:

Greatly simplify configuration choices:

* The distinction between "large" and "small" memory sizes has been
  eliminated; nowadays this makes little sense, since our old notion
  of "large" is tiny compared to modern programs.  The "--large"
  option is now accepted but ignored.  The old MITSCHEME_LARGE_xxx and
  MITSCHEME_SMALL_xxx environment variables are ignored, replaced by
  two new vars MITSCHEME_HEAP_SIZE and MITSCHEME_STACK_SIZE.  (There's
  no var for constant size since it's rarely necessary to specify it.)

* The default heap size is set at 4 megawords, much larger than our
  previous "large" size, and adequate for general use.

* We use the "all.com" band by default.  The "--compiler" and
  "--edwin" options are now accepted but ignored.  The env vars
  MITSCHEME_COMPILER_BAND, MITSCHEME_EDWIN_BAND, and
  MITSCHEME_ALL_BAND are now ignored.