Implementing ISA

[Leopold Toetsch]

When looking at vtables (and *.ops) I'm missing C<isa>.

  isa(out INT, in PMC, im STR)  # set $1 to 1 if class $2 isa $3.
  isa(out PMC, in PMC)
      # set $1 to a const array containing $2's classes (parent first)
  new P0, .PerlArray
  isa P1, P0 # ("PerlArray", "Array", "array")
The last entry ("array") is generated by the system out of some flags. 
This is the general type of the PMC (e.g. array, hash, sub, scalar, 
struct), so that code like this works:

  new P5, .PerlArray
  invoke
  ...
  isa I0, P5, "array"
  if I0, yep_has_array_interface
Comments welcome
leo