Custom lexers can now terminate operators

New constraint: custom lexer output is dropped whenever it is used to terminate an operator nested inside another custom lexer, because the recursive call has to return exactly one lexeme

orchid-base/src/tl_cache.rs

@@ -6,4 +6,18 @@ macro_rules! tl_cache {
 		}
 		V.with(|v| v.clone())
 	}};
+	(async $ty:ty : $expr:expr) => {{
+		type CellType = std::cell::OnceCell<$ty>;
+		thread_local! {
+			static V: CellType = std::cell::OnceCell::default();
+		}
+		match V.with(|cell: &CellType| cell.get().cloned()) {
+			Some(val) => val as $ty,
+			None => {
+				let val = $expr;
+				let _ = V.with(|cell: &CellType| cell.set(val.clone()));
+				val as $ty
+			},
+		}
+	}};
 }