make types within generate_solution_type macro explicit (#8447)

* make types within `generate_solution_type` macro explicit

Closes #8444.

Just changes the parsing logic for that macro; does not change any
emitted code. The associated types associated with the macro now
require explicit, keyword-style declaration.

**Old**:

```rust
sp_npos_elections::generate_solution_type!(
	#[compact]
	pub struct TestCompact::<VoterIndex, TargetIndex, PerU16>(16)
);
```

**New**:

```rust
sp_npos_elections::generate_solution_type!(
	#[compact]
	pub struct TestCompact::<VoterIndex = VoterIndex, CandidateIndex = TargetIndex, Accuracy = PerU16>(16)
);
```

* un-ignore doc-tests

* use new form in bin/node/runtime/

* rename CandidateIndex -> TargetIndex

* add tests demonstrating some potential compile failures

substrate/frame/election-provider-multi-phase/src/mock.rs

@@ -65,7 +65,7 @@ pub(crate) type TargetIndex = u16;
 
 sp_npos_elections::generate_solution_type!(
 	#[compact]
-	pub struct TestCompact::<VoterIndex, TargetIndex, PerU16>(16)
+	pub struct TestCompact::<VoterIndex = VoterIndex, TargetIndex = TargetIndex, Accuracy = PerU16>(16)
 );
 
 /// All events of this pallet.