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`?

lib/binding_rust/wasm_language.rs

@@ -133,7 +133,7 @@ impl Drop for WasmStore {
 }
 
 impl fmt::Display for WasmError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let kind = match self.kind {
             WasmErrorKind::Parse => "Failed to parse Wasm",
             WasmErrorKind::Compile => "Failed to compile Wasm",