Include type of missing trait methods in error

[aij]

Feature request (improved diagnostics):

This is what the error currently looks like:

src/sha256.rs:48:1: 50:2 error: not all trait items implemented, missing: `fmt` [E0046]
src/sha256.rs:48 impl core::fmt::Display for Sha256 {
src/sha256.rs:49 
src/sha256.rs:50 }
error: aborting due to previous error

In order to implement the missing method I need to double check the type in the documentation, even though I already know what the function needs to do. It would be really convenient if the compiler could print out the required type for me.