[RfC] put pmc->data into PMC_EXT

[Leopold Toetsch]

With the last bunch of changes (PMC-data-4) the infrastucture is ready - 
almost:

There are some places in the code, where not a specific PMC of some 
class is allocated, but only a call to new_pmc_header is done. These are:
- newinterp
- loadlib
- io/io.c:new_io_pmc
All of course assume, that they can stuff something into pmc->data.

I think its time, to make real PMC classes for these 3 fake classes.

Or/And: don't use pmc->data but pmc->cache->struct_val as storage for 
the desired item.

Comments welcome,
leo