assign opcodes

[Leopold Toetsch]

I hopefully got the semantics of assign Px,Py right now. The LHS gets 
the value of RHS, eventually morphing itself to the source type.

Anyway:

  assign Px, {Iy,Sy,Ny}

are not needed IMHO, these end up as set_<type>_native and are identical 
to set Px, {Iy,Sy,Ny}.

But we are missing keyed variants to set the value of some aggregate member:

  assign(in PMC, in KEY, in PMC)
  assign(in PMC, in INTKEY, in PMC)
with the 2 vtables assign_keyed and assign_keyed_int.

Comments welcome
leo