lambdas and blocks should both have optional type decls

[nikomatsakis]

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.

[marijnh]

I took a stab at making types optional for other function expressions, but I just don't understand the type checker well enough to get that to behave.

[nikomatsakis]

I am looking in this area. I may give it a go but I am going to get sendable functions working first.

[catamorphism]

@nikomatsakis - Is this obsolete? I don't seem to recall either one requiring type decls recently...

[nikomatsakis]

Not yet obsolete, but I think I may have opened a second bug on this issue at some point.

[nikomatsakis]

Closing in favor of #2093, which is more precise.