* [REFACTOR] Polish ffi convention.

- Remove the src/api, keep registration local to the c++ function.
- Remove the api_internal as it is no longer needed.

* Update the codebase walk through

* [REFACTOR] Move support related code to include/tvm/support

- tvm/logging.h -> tvm/support/logging.h
- remove tvm/base.h, move with into tvm/support/with.h

* src/common -> src/support