[RfC] key strings/numbers/PMCs

[Leopold Toetsch]

Currently there is no simple way, to packout a Key that has number or 
string key members. PackFile_Constant_pack() for PFC_KEY does a linear 
lookup (find_in_const) to get at the index of the string or number in 
the constant table. This is really ugly.

So I'd change that, so that key string/numbers (and PMCs) don't have the 
actual string/number/PMC inside, but the constant table index of the 
item. This avoids the linear search for the index.

This also means, that key_new_cstring (and _number, if one needs that) 
generates a constant table entry and that key_string returns the item 
from the constant table[1]. Now to avoid many duplicates it would be 
best to cache these entries and do a hash lookup first.

This all is also related with the discussion of constant PMCs/STRINGs, 
we had some time ago "[RfC] constant PMCs and classes" and with the lack 
of "get_string_keyed_str" and friends for hash lookup.

E.g. pmc_register() currently generates 3 (three) intermediate key PMCs 
for hash lookup in the classname_hash(). The class and object code has 
such lookup all over the place and it will be more in the long run.

Comments welcome,
leo
[1] in combination with the PObj_constant_FLAG - a really dynamic 
variant would work like now, albeit by far the most usage of such keys 
is truely constant.