This commit merges the current development state of the MCD client stub

1. **Remote Procedure Call (RPC)**

* implement complete MCD API in `src/mcdstub.cpp`
* refactor request-response communication with the server in
  `src/comm.cpp`
* add custom RPC framework in `include/mcd_rpc.h` to simplify
  server-side integration

2. **Adapter Functionality**

* define customizable adapter architecture in `include/adapter.hpp`
* add default "passthrough" adapter in `src/core_passthrough.cpp`

3. **RPC Code Generator**

* semantically define all MCD types in `codegen/structs.py`
* add (un)marshal code generator in `codegen/main.py`

4. **Test Framework**

* wrap complete MCD API into ctypes in `tests/mcdapi/`
* add unit tests for several MCD functions and several targets (QEMU)
* support automatic launch of QEMU from Linux and Windows