Work around mypy limitations - #191

Currently mypy doesn't interpret things other than classes as being
compatible with `Type[T]` (they literally have to be instances of
`type`). This makes it hard to properly type the `decode` methods or
`Decoder` constructors, as passing type-like-things (e.g. `Union[int,
str]`) will error under mypy. `pyright` doesn't have this limitation.

For now we add a fallback `overload` to these methods that infers the
decoder type as `Decoder[Any]`. This means that `mypy` will no longer
error, but will require an explicit annotation to infer the type of the
output of `decode`.

If/once the `TypeForm` PEP lands, this hack can be removed. Until then
`pyright` support for these annotations will be much better than `mypy`
support.