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/compact/tests/ui/fail/missing_accuracy.rs

@@ -0,0 +1,9 @@
+use sp_npos_elections_compact::generate_solution_type;
+
+generate_solution_type!(pub struct TestSolution::<
+	VoterIndex = u16,
+	TargetIndex = u8,
+	Perbill,
+>(8));
+
+fn main() {}

substrate/primitives/npos-elections/compact/tests/ui/fail/missing_accuracy.stderr

@@ -0,0 +1,5 @@
+error: Expected binding: `Accuracy = ...`
+ --> $DIR/missing_accuracy.rs:6:2
+  |
+6 |     Perbill,
+  |     ^^^^^^^

substrate/primitives/npos-elections/compact/tests/ui/fail/missing_target.rs

@@ -0,0 +1,9 @@
+use sp_npos_elections_compact::generate_solution_type;
+
+generate_solution_type!(pub struct TestSolution::<
+	VoterIndex = u16,
+	u8,
+	Accuracy = Perbill,
+>(8));
+
+fn main() {}

substrate/primitives/npos-elections/compact/tests/ui/fail/missing_target.stderr

@@ -0,0 +1,5 @@
+error: Expected binding: `TargetIndex = ...`
+ --> $DIR/missing_target.rs:5:2
+  |
+5 |     u8,
+  |     ^^

substrate/primitives/npos-elections/compact/tests/ui/fail/missing_voter.rs

@@ -0,0 +1,9 @@
+use sp_npos_elections_compact::generate_solution_type;
+
+generate_solution_type!(pub struct TestSolution::<
+	u16,
+	TargetIndex = u8,
+	Accuracy = Perbill,
+>(8));
+
+fn main() {}

substrate/primitives/npos-elections/compact/tests/ui/fail/missing_voter.stderr

@@ -0,0 +1,5 @@
+error: Expected binding: `VoterIndex = ...`
+ --> $DIR/missing_voter.rs:4:2
+  |
+4 |     u16,
+  |     ^^^

substrate/primitives/npos-elections/compact/tests/ui/fail/no_annotations.rs

@@ -0,0 +1,9 @@
+use sp_npos_elections_compact::generate_solution_type;
+
+generate_solution_type!(pub struct TestSolution::<
+	u16,
+	u8,
+	Perbill,
+>(8));
+
+fn main() {}

substrate/primitives/npos-elections/compact/tests/ui/fail/no_annotations.stderr

@@ -0,0 +1,5 @@
+error: Expected binding: `VoterIndex = ...`
+ --> $DIR/no_annotations.rs:4:2
+  |
+4 |     u16,
+  |     ^^^

substrate/primitives/npos-elections/compact/tests/ui/fail/swap_voter_target.rs

@@ -0,0 +1,9 @@
+use sp_npos_elections_compact::generate_solution_type;
+
+generate_solution_type!(pub struct TestSolution::<
+	TargetIndex = u16,
+	VoterIndex = u8,
+	Accuracy = Perbill,
+>(8));
+
+fn main() {}

substrate/primitives/npos-elections/compact/tests/ui/fail/swap_voter_target.stderr

@@ -0,0 +1,5 @@
+error: Expected `VoterIndex`
+ --> $DIR/swap_voter_target.rs:4:2
+  |
+4 |     TargetIndex = u16,
+  |     ^^^^^^^^^^^