Rollup merge of #64910 - Centril:params-cleanup, r=petrochenkov

syntax: cleanup param, method, and misc parsing

Do some misc cleanup of the parser:
- Method and parameter parsing is refactored.
- A parser for `const | mut` is introduced that #64588 can reuse.
- Some other misc parsing.

Next up in a different PR:
- ~Implementing #64252 -- maybe some other time...
- Heavily restructuring up `item.rs` which is a mess (hopefully, no promises ^^).

r? @petrochenkov

src/libsyntax/parse/diagnostics.rs

@@ -1180,7 +1180,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    crate fn expected_semi_or_open_brace(&mut self) -> PResult<'a, ast::TraitItem> {
+    crate fn expected_semi_or_open_brace<T>(&mut self) -> PResult<'a, T> {
         let token_str = self.this_token_descr();
         let mut err = self.fatal(&format!("expected `;` or `{{`, found {}", token_str));
         err.span_label(self.token.span, "expected `;` or `{`");