Mapping type is not well supported in expressions

[vmagamedov]

When we have a function definition which accepts Mapping[String, Any], and we pass an option's value with the same type, currently we get this error:

Traceback (most recent call last):
  ...
  File "/usr/local/lib/python3.7/dist-packages/hiku/sources/graph.py", line 57, in __postprocess__
    reqs = RequirementsExtractor.extract(self.sub_graph.types, expr)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/refs.py", line 122, in extract
    extractor.visit(expr)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/refs.py", line 131, in visit
    super(RequirementsExtractor, self).visit(node)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/nodes.py", line 71, in visit
    node.accept(self)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/nodes.py", line 40, in accept
    return visitor.visit_tuple(self)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/refs.py", line 143, in visit_tuple
    self.visit(arg)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/refs.py", line 128, in visit
    req = ref_to_req(self._types, ref)
  File "/usr/local/lib/python3.7/dist-packages/hiku/expr/refs.py", line 82, in ref_to_req
    assert not isinstance(ref_type, _CONTAINER_TYPES), ref_type
AssertionError: Mapping[String, Any]