typing: add type annotations for wireprotoframing encoder/decoder classes This was low hanging fruit, and will point out an apparent programming bug. The arguments are left untyped for now, because the internal compressors and decompressors generally take a `Buffer` object, and return bytes. I've had issues around using `typing_extensions.Buffer` with pytype in the past, but also the identity classes return the argument unmodified, so that will break the type annotation. We'd probably need an instance check, and a cast if it's not bytes. Having to roll up all of the encoder and decoder types into a union type is a bit annoying, but the protocol class can't be used in the map because it then assumes that type will be instantiated. I also couldn't get it to work with a `TypeVar` bound to indicate that it's actually a subclass either. Oh well.