base scalar PMC semantics

[Leopold Toetsch]

First, there was some dicussion not too long ago:
 Subject: Numeric semantics for base pmcs [1]
 Subject: Last bits of the basic math semantics
The current Integer PMC doesn't yet follow the results of these threads.
Basic behavior of that type is Perl6 or Python semantics, which is: it's 
basically an arbitrary precision integer, like Python's int/long type 
after merging. To achieve this functionality it silently morphs results 
to a Big type capable of doing the arbitrary precision.

The summary in [1] also mentions type coercion:
10) The destination PMC is responsible for final conversion of the 
inbound value

E.g. when we have
    MMD add(PyInt + PyInt)
a) no overflow: VTABLE_set_integer_native(interp, dest, the_sum)
the set_integer_native vtable is responsibe to convert the C<dest> PMC 
into a PyInt. For Perl types it'll be PerlInt. And base PMCs use 
Integer. Following strictly this scheme does allow the inheritance of 
all common functionality.

b) overflow:
   if (self == dest) {
      VTABLE_set_bignum(interp, self, self.intval)
      // redispatch
   }
   else {
      VTABLE_set_bignum(interp, dest, self.intval)
      temp = new dest.type
      VTABLE_set_bignum(interp, temp, self.intval)
      // redispatch
or a similar scheme.
Float and String needs the same refactoring, but that's simpler.
To use that functionality we need a better notion for multiple 
inheritance inside PMCs.

  PerlInt isa (PerlAny, Integer)
  PyInt   isa (PyObject, Integer)
Comments?
leo