Make all constant folding functions static to oeval.c.
Handle parse time constant fold exceptions.

  The code is now aware of exception handling, so, if an exception
is generated, do not change input neither abort.
Rewrite constant folding code to use ovm_xyz functions.

  This removes a large amount of duplicate and complex code,
that could previously hide bugs in special conditions.
  Now, other than the optimizations for small/typed values in
oemit*.c, there is only one implementation of math functions.