Use simpler abbreviations when reporting error types

[sophiajt]

Given the following code:

Ideally the output should look like:

Rather than printing the full types to the user, perhaps we want to have a kind of type abbreviation for readability.

The closure type [closure@f52.rs:13:18 13:21] is not one the user can type in, nor likely one they'll even track down in the source code. We could potentially just say closure (and specify it is unique in some way)

For the std::slice::Iter, I wonder if this is some we might abbreviate for readability. Granted Iter by itself is ambiguous, but perhaps there's a simpler way we could write this to allow more space for other information to the user.