Drop `msgpack` dependency, go full `msgspec`

[goodboy]

In prep for #196, #311, and general use of py 3.10's match: syntax I think we should fully embrace msgspec as our primary interchange lib.

• It's been tested thoroughly in production apps and performed great
• we can't really move to a typed messaging model without having native support for msgspec.Struct

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Dev/Release plan:

• drop msgpack as part of our next alpha 0.1.0a5
• refactor the ._ipc transport types and change the MsgpackTCPStream to be msgspec based.