Support ? operator in for loops

[estebank]

Should the following be allowed?

for item? in my_iter {
    ...
}

Every now and then I encounter the need to iterate over Results and end up having to write the following, which isn't onerous but is not ergonomic:

for item in my_iter {
    let item = item?;
    ...
}

[ids1024]

This has been requested before: rust-lang/rfcs#2012

[estebank]

@ids1024 thanks for linking to it! I'll close this ticket in favor of that RFC.