Better
Remove `StructuredValidationError`
FEATURE `_validate_DictOf`

`_validate_DictOf` uses the new validation pattern.
Finish tests
Done: `_validate_DictOf`
xxx
FEATURE `_validate_list`
Done: `_validate_list`
FEATURE `_validate_ListOf`
Done: `_validate_ListOf`
FEATURE Sequence, mapping validation

Use abstract base classes to validate sequence and mapping types. When
`schema` is a `list` or `dict`, any sequence or mapping type
(respectively) will pass.
Test `_validate_scalar`
Done: `_validate_scalar`
FEATURE `Nullable` rule

Add `Nullable` rule. Allows `None` values to pass validation.
Done: `Nullable`
FEATURE `_validate_tuple`
Test: `_validate_tuple`
Done: `validate_tuple`
MISC Organization
FEATURE `raise_type_validation_error`