add more core validators

[Defunctionalize]

There are a couple more validator types that should be added to the core functionality in order to be ready for 1.0

first one I can think of off the top of my head is:

• support for optional keys in dictionary. the api should be something like

non_strict_map(
    {"required_key_1": expected_value_validator,
     "required_key_2": expected_value_validator,},
    optional={"optional_key_1": expected_value_validator,
              "optional_key_2": expected_value_validator}
)