lazyindex: speed up __getitem__
This function is fairly performance sensitive, so we make a couple
ugly tweaks:
- keep all entries packed so we needn't test entry types
- fold index lookup/load into unpack call to eliminate
local variable setting
# should print nothing
# should print ? b/x
? b/x
# should print A b/x
A b/x
# should forget b/x
forgetting b/x
# should print nothing