[CVS ci] constant PMCs

[Leopold Toetsch]

As already mentioned in the DOD threads, this patch adds a new constant_pmc_pool and one additional interface function:
PMC *constant_pmc_new_noinit(Interp*, INTVAL base_type)

Constant PMCs are used by key constants (generated during PBC reading in packfile.c) and in PerlHash for a global undef object.
Probably more places for usage would be Array/PerlArray for returning undefs (but not for storing them in the array).

Also included: more flags cleanup, constant PMCs are no more checked during DOD.

leo