PBC, Const Tables, and STRINGs

[chromatic]

The current version of PBC uses constant tables for PIR artifacts such as 
literal strings in PIR.

It doesn't use constant tables for data such as Sub metadata.  While NQP can 
generate closures and inner blocks fairly efficiently, we have to keep extra 
constant STRINGs around in the system because our freeze/thaw mechanism 
doesn't know how to store constant attributes of Subs in the packfile constant 
tables.

We could reduce PBC size, fakecutable size, freezing time, thawing time, and 
runtime memory use (not to mention GC runs) by coalescing these attributes 
into PBC constants somehow.

Rough figures on Rakudo startup show some 405,000 STRINGs thawed from PBC, 
while some 16,000 are unique.  That's 95% waste.

Perhaps the ImageIO PMC's shift/unshift string VTABLEs could manage these 
constant STRINGs appropriately.

-- c