Solve reduce/reduce conflict in patterns. 

### AST:
* Drop `ConidPattern`. Instead, allow `Pattern` to have no argument list.

### Parse
* Split `pattern` rule into `complex_pattern` and `simple_pattern`.
* Replace `simple_pattern_list` with `simple_pattern_seq`.
* A `complex_pattern` is not allowed to have zero arguments. This solves the ambiguity.

### Parse test:
* Fix regression test.
* Expand testing of `simple_pattern_seq`.