Create 3-arg version of lookup
Currently, lookup comes in only the 2-arg flavor (i.e. gimme a key and I'll give you a value or nil). The issue with this is that nil is potentially a legal value. Therefore, the 3-arg version logic is the same as the 3-arg `assoc` (.lookup this key not-found).
This change should propagate to all of the existing cache impls.
Closing out all resolved issues as of 0.6.5 in order to track the next release.
Added in f4450a039ef703ce62c61dd497aafed73194f1c2