fleshing out extend.h

[Mitchell N Charity]

Pmc class headers providing parts of parrot's C extension api are not
being included in extend.h. Such as the generated file
./classes/pmc_array.h, with its Parrot_Array_push_integer. Such files
are not being included anywhere in include/parrot/ at all.

So it looks like a ./config/gen/ generator is needed. Perhaps using
Configure::Data->get('pmc_names') to generate a bunch of #include's
for a new header file.

One might thus create a ./include/parrot/pmc_apis.h, say, to be
included by extend.h. [1]

Does this seem a plausible way to proceed?

Mitchell
[1] extend.h is already including several other parrot headers.