Generalize core #3129

Currently the acter-core-rust crate is bound to the matrix-sdk (for client things), ruma (for serialization) and through ruma to the jsint for parsing the matrix int types.

We want to split this into a few parts:

• acter-core, which is transport and serialization agnostic, that contains the main engine, runtime and store and signaling system in a generalized form
• acter-models, which implements our specific types, with the indices and stuff, which describes the basic types, the fields and specific information
• acter-matrix, which implements the models and core for our specific ruma and matrix types, providing the api layer that acter-core provides today but allowing us to provider other transport and serialization mechanism as well

Best way going forward is probably:

• renaming acter-core to acter-matrix
• start extracting and generalizing the engine and models into their respective crates and types.