TODO

basic interface to the hosting C program
  (with wren_ prefixes & header file)
better error reporting and recovery
warn when memory gets low
optional preloaded standard library
  (not in RAM -- I suppose we'd interpret the high bit of our 16-bit
  pointers as indicating it's in ROM space)
write a bigger example, something vaguely useful
fix remaining XXXs
tauten and clean up the code
optionally compile VM code into flash (I guess this'd obviate the 
  separate ROM addressing, above)
make it easier to add/remove primitives
improve safety in the face of pokes
measure size of VM-compiled programs
make VM encoding a bit more compact (the easy stuff)
check for keyboard interrupt (or equivalent)
basic debugging support
  * backtrace, at least
  * 'panic' primitive
  * decompiler?

do something about (-1) as a literal? (parens ugly) - DONE - If last instruction is push, just negate.
make 'else' clause optional? 
require some end-of-function delimiter to reduce scope for ambiguity?
  (especially the treatment of newlines)

a cleverer VM encoding vaguely along the lines of 'Little Smalltalk':
  * 4-bit split (opcode-family, parameter)
  * two 'misc' families with the opcode in the parameter part
  * extended-parameter families 
    (with 4-bit family code followed by 1- or 2-byte parameter value)
  * the obvious families like PUSH, FETCH_LOCAL, BRANCH, JUMP
  * the remaining space can go to combined superinstructions
    (interpreted from a 16-byte table for each such family; the
    peephole optimizer uses the same tables)
along with:
  a 2-pass compiler, comprising:
    * parse and generate intermediate code forwards
      (intermediate code is pretty much like the current simple instruction
      set, but designed to be read only from the end backwards)
    * peephole-optimize backwards, converting to actual VM code
    * (and finally, memmove() the VM code into place; it will have shrunk)
  a self-hosted compiler, to take advantage of the above
    (it should be smaller overall than the C-compiled compiler)
syntax extensible at runtime, using the above
closures allocated into the dictionary, a la Forth's CREATE DOES>
nested 'let' and 'fun'