disallow '.' as mapsTo token (only ↦ now)

Intention is to reuse the '.' token as namespace delimiter
and '::' to be used as return type prefix instead

src/parser.rs

@@ -309,7 +309,7 @@ where
                     tokens.next();
 
                     let mut variable_bindings = parse_binding_expr(typectx, tokens)?;
-                    let _ = parse_expect(tokens, LTIRToken::LambdaBody);
+                    let _ = parse_expect(tokens, LTIRToken::MapsTo);
                     let body = parse_expr(typectx, tokens)?;
 
                     return Ok(LTExpr::Abstraction {