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/primitives/npos-elections/fuzzer/src/compact.rs

@@ -4,7 +4,11 @@ use sp_npos_elections::sp_arithmetic::Percent;
 use sp_runtime::codec::{Encode, Error};
 
 fn main() {
-	generate_solution_type!(#[compact] pub struct InnerTestSolutionCompact::<u32, u32, Percent>(16));
+	generate_solution_type!(#[compact] pub struct InnerTestSolutionCompact::<
+		VoterIndex = u32,
+		TargetIndex = u32,
+		Accuracy = Percent,
+	>(16));
 	loop {
 		fuzz!(|fuzzer_data: &[u8]| {
 			let result_decoded: Result<InnerTestSolutionCompact, Error> =