provide get_slot_for for attributes
cache lookups for attribute reading and writing

this is probably still broken if we add things to the attribute after
the class is created and used - we need a way to clear these caches in
FINALIZE
after:FETCH_DATA should get the data that was fetched
drop attribute slot caches when the class changes
use the class name instead of refaddr for slot cache
factor this out
custom op for storing the invocant in a predictable place

otherwise we have to pass around the name of the invocant everywhere,
because it's user-specifiable
cleanups
fix B::Deparse
this really belongs in ::syntax
clean up method opcode generation
remove this unnecessary ANONLIST
pad_setsv only exists in debug builds
autovivification is also a develop requirement