Override diagnostic::on_unimplemented message for all serde_core traits

[dtolnay]

This prevents diagnostics being rendered like serde_core::ser::Serialize and serde_core::de::Deserialize in projects that have no direct dependency on serde_core.

- error[E0277]: the trait bound `MyStruct: serde_core::ser::Serialize` is not satisfied
+ error[E0277]: the trait bound `MyStruct: serde::Serialize` is not satisfied
   --> src/main.rs:5:27
    |
  5 |     serde_json::to_string(&thing);
    |     --------------------- ^^^^^^ unsatisfied trait bound
    |     |
    |     required by a bound introduced by this call

The attributes make error messages arguably sometimes worse than pre-serde_core by always rendering serde::Serialize and never Serialize when use serde::Serialize is in scope, which rustc's default message construction knows to recognize. But this is probably negligible.

- error[E0277]: the trait bound `MyStruct: Serialize` is not satisfied
+ error[E0277]: the trait bound `MyStruct: serde::Serialize` is not satisfied
   --> src/main.rs:5:27
    |
  5 |     serde_json::to_string(&thing);
    |     --------------------- ^^^^^^ unsatisfied trait bound
    |     |
    |     required by a bound introduced by this call

I explored the alternative of setting [lib] name = "serde" in serde_core/Cargo.toml which also prevents serde_core::ser::Serialize in messages, but has the unwanted effect of also inserting the following noise into every such error:

note: there are multiple different versions of crate `serde` in the dependency graph
    --> $WORKSPACE/serde_core/src/ser/mod.rs:225:1
     |
 225 | pub trait Serialize {
     | ^^^^^^^^^^^^^^^^^^^ this is the required trait
     |
    ::: src/main.rs:1:1
     |
   1 | struct MyStruct;
     | --------------- this type doesn't implement the required trait
...
   4 |     let _ = serde_json::to_string(&MyStruct);
     |             ----------
     |             |
     |             one version of crate `serde` used here, as a dependency of crate `serde`
     |             one version of crate `serde` used here, as a dependency of crate `serde_json`
     |
    ::: $WORKSPACE/serde/src/private/de.rs:2347:1
     |
2347 | pub trait IdentifierDeserializer<'de, E: Error> {
     | ----------------------------------------------- this is the found trait
     = help: you can use `cargo tree` to explore your dependency tree

The "this is the found trait" pointing to IdentifierDeserializer seems like a compiler bug...