PMC constants

[Leopold Toetsch]

While trying to speed up hash lookups [1] I came (again) to the problem 
that we are missing true PMC constants. We just have a special Sub PMC 
for storing subroutine entries but no general way to represent a 
constant PMC item. E.g.:

   .const pmc s = "value"
   .const pmc i = 1
   ...
   add $P0, i
   setprop $P0, "xxx", s
This would AFAIK not need a lot more opcode variants, because where we 
now have C<add_p_ic> and C<add_p_nc>, we would then just have 
C<add_p_pc>[2]. Only some ops that take PMCs only would need additional 
variants like setprop_p_sc?_pc.
I'm currently thinking of constant scalar PMCs only, but this could get 
generalized to constant aggregates too.

Comments?
leo
[1] constant _S("string") gets a precomputed hash value
[2] Compiling "add P0, 1" imcc would look for add_p_ic and if this isn't 
there then for add_p_pc *and* convert the constant.