lambdas and blocks should both have optional type decls #1275
Description
Right now, lambdas must declare types for their arguments and blocks must not. But both are represented in more-or-less the same way and seem to go through a similar inference path in typeck.rs; I think we ought to make parameter types optional in both cases. Naturally, if the lambda/block is not used in an argument position, or at least being assigned to a variable of known type, we will not be able to infer a type for the arguments and so an error would occur.