Rust: Type inference for for loops and array expressions

[geoffw0]

Adds type inference for for loops and array expressions. Currently for for loops this is limited to loops iterating through arrays.

@hvitved please advise.

• I think for the more general cases we will need to decode the Iterator type and perhaps understand IntoIterator.
• I'm not sure whether I should be attempting to model implicit dereferences, or perhaps a more general solution for that problem might be in the works?