fix: Naming; Lifetimes; deny(rust_2018_idioms)

- One has to think about lifetimes if a type has one:
  - `<&'a Node<'tree>>::language` now returns `Language<'tree>` instead of
    `Language<'a>`, as it should;
- Renamed `struct TreeCursor<'cursor>` into `struct TreeCursor<'tree>`,
  to be consistant with the usages and reduse confusion;
- Remove explicit "outlives" requirements from `QueryMatches`, `QueryCaptures`,
  and their impl blocks, because they're inferred
- TODO: should `'query` be renamed into `'cursor`?

crates/cli/src/init.rs

@@ -1188,7 +1188,7 @@ fn generate_file(
     path: &Path,
     template: &str,
     language_name: &str,
-    generate_opts: &GenerateOpts,
+    generate_opts: &GenerateOpts<'_>,
 ) -> Result<()> {
     let filename = path.file_name().unwrap().to_str().unwrap();