When a trait is not implemented for a type, but there *is* an `impl`
for another type or different trait params, we format the output to
use highlighting in the same way that E0308 does for types.
The logic accounts for 3 cases:
- When both the type and trait in the expected predicate and the candidate are different
- When only the types are different
- When only the trait generic params are different
For each case, we use slightly different formatting and wording.
Change the implicit `Sized` `Obligation` `Span` for call expressions to
include the whole expression. This aids the existing deduplication
machinery to reduce the number of errors caused by a single unsized
expression.
