Override diagnostic::on_unimplemented message of all serde_core traits

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.

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.

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...

test_suite/tests/ui/unimplemented/required_by_dependency.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `MyStruct: serde_core::ser::Serialize` is not satisfied
+error[E0277]: the trait bound `MyStruct: serde::Serialize` is not satisfied
  --> tests/ui/unimplemented/required_by_dependency.rs:4:35
   |
 4 |     serde_test::assert_ser_tokens(&MyStruct, &[]);
@@ -32,7 +32,7 @@ note: required by a bound in `assert_ser_tokens`
   |     T: ?Sized + Serialize,
   |                 ^^^^^^^^^ required by this bound in `assert_ser_tokens`
 
-error[E0277]: the trait bound `MyStruct: serde_core::de::Deserialize<'_>` is not satisfied
+error[E0277]: the trait bound `MyStruct: serde::Deserialize<'de>` is not satisfied
  --> tests/ui/unimplemented/required_by_dependency.rs:5:34
   |
 5 |     serde_test::assert_de_tokens(&MyStruct, &[]);

test_suite/tests/ui/unimplemented/required_locally.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `MyStruct: Serialize` is not satisfied
+error[E0277]: the trait bound `MyStruct: serde::Serialize` is not satisfied
   --> tests/ui/unimplemented/required_locally.rs:21:15
    |
 21 |     to_string(&MyStruct);
@@ -32,7 +32,7 @@ note: required by a bound in `to_string`
  6 |     T: Serialize,
    |        ^^^^^^^^^ required by this bound in `to_string`
 
-error[E0277]: the trait bound `MyStruct: Deserialize<'_>` is not satisfied
+error[E0277]: the trait bound `MyStruct: serde::Deserialize<'de>` is not satisfied
   --> tests/ui/unimplemented/required_locally.rs:22:23
    |
 22 |     let _: MyStruct = from_str("");

test_suite/tests/ui/with/incorrect_type.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+error[E0277]: the trait bound `&u8: serde::Serializer` is not satisfied
   --> tests/ui/with/incorrect_type.rs:14:10
    |
 14 | #[derive(Serialize, Deserialize)]
@@ -30,7 +30,7 @@ note: function defined here
  9 |     pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
    |            ^^^^^^^^^
 
-error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+error[E0277]: the trait bound `&u8: serde::Serializer` is not satisfied
   --> tests/ui/with/incorrect_type.rs:15:25
    |
 15 | struct W(#[serde(with = "w")] u8, u8);
@@ -49,7 +49,7 @@ error[E0308]: `?` operator has incompatible types
    |
    = note: `?` operator cannot convert from `()` to `u8`
 
-error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+error[E0277]: the trait bound `&u8: serde::Serializer` is not satisfied
   --> tests/ui/with/incorrect_type.rs:17:10
    |
 17 | #[derive(Serialize, Deserialize)]
@@ -81,7 +81,7 @@ note: function defined here
  9 |     pub fn serialize<T, S: Serializer>(_: S) -> Result<S::Ok, S::Error> {
    |            ^^^^^^^^^
 
-error[E0277]: the trait bound `&u8: Serializer` is not satisfied
+error[E0277]: the trait bound `&u8: serde::Serializer` is not satisfied
   --> tests/ui/with/incorrect_type.rs:18:35
    |
 18 | struct S(#[serde(serialize_with = "w::serialize")] u8, u8);