Various tests for rewriting Beancount in C++.

We'd like to have a working environment with the following tools integrated and
working well together:

- bazel
- kythe

C++:
- abseil
- protocol buffers
- gtest
- gmock
- Some decimal number representation
- flex
- bison
- grpc
- riegeli

Python:
- python3 development
- protocol buffers (fast cpp for Python)
- pytest
- pytype
- pylint
- clif or cython
- protoptr (from nucleus)
