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/src/tests.rs

@@ -1148,7 +1148,11 @@ mod solution_type {
 
 	type TestAccuracy = Percent;
 
-	generate_solution_type!(pub struct TestSolutionCompact::<u32, u8, TestAccuracy>(16));
+	generate_solution_type!(pub struct TestSolutionCompact::<
+		VoterIndex = u32,
+		TargetIndex = u8,
+		Accuracy = TestAccuracy,
+	>(16));
 
 	#[allow(dead_code)]
 	mod __private {
@@ -1158,7 +1162,7 @@ mod solution_type {
 		use sp_arithmetic::Percent;
 		generate_solution_type!(
 			#[compact]
-			struct InnerTestSolutionCompact::<u32, u8, Percent>(12)
+			struct InnerTestSolutionCompact::<VoterIndex = u32, TargetIndex = u8, Accuracy = Percent>(12)
 		);
 	}
 
@@ -1166,7 +1170,11 @@ mod solution_type {
 	fn solution_struct_works_with_and_without_compact() {
 		// we use u32 size to make sure compact is smaller.
 		let without_compact = {
-			generate_solution_type!(pub struct InnerTestSolution::<u32, u32, Percent>(16));
+			generate_solution_type!(pub struct InnerTestSolution::<
+				VoterIndex = u32,
+				TargetIndex = u32,
+				Accuracy = Percent,
+			>(16));
 			let compact = InnerTestSolution {
 				votes1: vec![(2, 20), (4, 40)],
 				votes2: vec![
@@ -1180,7 +1188,11 @@ mod solution_type {
 		};
 
 		let with_compact = {
-			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));
 			let compact = InnerTestSolutionCompact {
 				votes1: vec![(2, 20), (4, 40)],
 				votes2: vec![