py/parse: Improve constant folding to operate on small and big ints.

Constant folding in the parser can now operate on big ints, whatever
their representation.  This is now possible because the parser can create
parse nodes holding arbitrary objects.  For the case of small ints the
folding is still efficient in RAM because the folded small int is stored
inplace in the parse node.

Adds 48 bytes to code size on Thumb2 architecture.  Helps reduce heap
usage because more constants can be computed at compile time, leading to
a smaller parse tree, and most importantly means that the constants don't
have to be computed at runtime (perhaps more than once).  Parser will now
be a little slower when folding due to calls to runtime to do the
arithmetic.