Add Bazel integration

[nak3]

Fixes https://github.com/google/autocxx/issues/478

This patch adds Bazel Integration.

It can build:

• autocxx-* libs/binaries - please see BUILD in the top directory.
• demo - please see demo/BUILD.

,which follows the structure of cxx project. So CI also built the demo app.

The commits contain a tons of file change but most of them are autogen files. The autogen and non-autogen files are separated so please refer to the commit message.

It's a good idea to open an issue first for discussion.

• [X] Tests pass
• [X] Appropriate changes to README are included in PR

[nak3]

Though I opened this PR I know I used some "hacky" way in demo/BUILD so I don't mind if this PR was refused or keep it open until the satisfied solution is found.