Suggest borrowing when it would satisfy an unmet trait bound

When there are multiple implementors for the same trait that is present
in an unmet binding, modify the E0277 error to refer to the parent
obligation and verify whether borrowing the argument being passed in
would satisfy the unmet bound. If it would, suggest it.

src/libcore/ops/function.rs

@@ -137,10 +137,6 @@ pub trait Fn<Args> : FnMut<Args> {
 #[rustc_paren_sugar]
 #[rustc_on_unimplemented(
     on(Args="()", note="wrap the `{Self}` in a closure with no arguments: `|| {{ /* code */ }}"),
-    on(
-        all(Args="(char,)", _Self="std::string::String"),
-        note="borrowing the `{Self}` might fix the problem"
-    ),
     message="expected a `{FnMut}<{Args}>` closure, found `{Self}`",
     label="expected an `FnMut<{Args}>` closure, found `{Self}`",
 )]