[PMC] init_pmc_props() DEPRECATED; init_pmc() GENERALIZED; beware set_pointer()

[Chip Salzenberg]

The documentation of the init_pmc() vtable method (a.k.a. '.sub __init_pmc'
for ParrotObjects) is inconsistent. In some places, the single PMC's
interpretation is simply described as "pmc-dependent"; in other places, it's
described as a set of properties, possibly keyed by number instead of name
[!].

The former usage is obviously more ... useful ... and will be consistently
documented as correct.

Meanwhile, I can't find any users of init_pmc_props (a.k.a. '.sub
__init_pmc_props' for ParrotObjects) so I'm going to deprecate it.

In a third topic, I seem to recall that some PMCs are using set_pointer() as
a dodge to avoid proper initialization, i.e. init() followed by
set_pointer() instead of a proper init_pmc(). If I'm remembering this
correctly, it's a bad idea.

In fact ... I'm not entirely sure I like that set_pointer() even exists,
since it's impossible to cleanly re-implement from PIR for a ParrotObject.
So I'll be reviewing whether it can go away.
--
Chip Salzenberg <ch...@pobox.com>