Experiment with a much more concise style

The style adopted in this branch is influenced by Arthur Whitney's A.
Minor changes to printing; whitespace
Whitespace
Minor word name changes
Minor changes to prompt
Fix a bug in the 2-arg type-checking macro
Define numeric words for floats and integers
Experimental syntax; add word names to runtime errors
On error, print the current executing word; whitespace
Add words for car, cdr, nilp
Experiment further with less horizontal whitespace
GC is now feature complete

   - GC is triggered only when necessary
   - Heaps are expanded if not enough memory is reclaimed
Minor macro changes
Whitespace
Fix a bug in the error-checking of cons
Remove obsolete REAMDE