Instead just instantiate `msgpack.Encoder/Decoder` instances inside
`mk_codec()` and assign them directly as `._enc/._dec` fields.
Explicitly take in named-args to both and proxy to the coder/decoder
instantiation calls directly.
Shuffling some codec internals:
- rename `mk_codec()` inputs as `ipc_msg_spec` and `ipc_pld_spec`, make
them mutex such that a payload type spec can't be passed if the
built-in msg-spec isn't used.
=> expose `MsgCodec.ipc_pld_spec` directly from `._dec.type`
=> presume input `ipc_msg_spec` is `Any` by default when no
`ipc_pld_spec` is passed since we have no way atm to enable
a similar type-restricted-payload feature without a wrapping
"shuttle protocol" ;)
- move all the payload-sub-decoders stuff prototyped in GH#311
(inside `.types`) to `._codec` as commented-for-later-maybe `MsgCodec`
methods including:
- `.mk_pld_subdec()` for registering
- `.enc/dec_payload()` for sub-codec field loading.
- also comment out `._codec.mk_tagged_union_dec()` as the orig
tag-to-decoder table factory, now mostly superseded by
`.types.mk_msg_spec()` which takes the generic parameterizing approach
instead.
- change naming to `types.mk_msg_spec(payload_type_union)` input, making
it more explicit that it expects a `Union[Type]`.
Oh right, and start exposing all the `.types.Msg` subtypes in the `.msg`
subpkg in prep for usage throughout the runtime B)
b6ed26589a