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/bin/node/runtime/src/lib.rs

@@ -519,8 +519,11 @@ parameter_types! {
 
 sp_npos_elections::generate_solution_type!(
 	#[compact]
-	pub struct NposCompactSolution16::<u32, u16, sp_runtime::PerU16>(16)
-	// -------------------- ^^ <NominatorIndex, ValidatorIndex, Accuracy>
+	pub struct NposCompactSolution16::<
+		VoterIndex = u32,
+		TargetIndex = u16,
+		Accuracy = sp_runtime::PerU16,
+	>(16)
 );
 
 impl pallet_election_provider_multi_phase::Config for Runtime {