Codebase cleanup

[k-ye]

We'd like to prioritize the productionization of the codebase. This is an umbrella issue. Below shows a growing list of the issues we'd like to address, which should bring the codebase to a much better state:

• Add googletest unit test framework [cleanup] Add googletest unit test framework #2195
• Untangle the dependencies between sub-modules [cleanup] Untangle the dependencies between the Taichi submodules #2196
• Add unit tests to a few critical modules, like IR and SNode
• Identify and remove the unused files as we go, possibly outdated files:
  • https://github.com/taichi-dev/taichi/blob/master/taichi/system/benchmark.h ?
  • https://github.com/taichi-dev/taichi/blob/master/taichi/common/interface.h ?
  • https://github.com/taichi-dev/taichi/blob/master/taichi/common/task.h ?
  • ... and more
• Replace T*_NAMESPACE_BEGIN (END) macros with namespace taichi { ... }.
• Header import order (may be useful for Reduce the C++ compilation duration #2203 )
• New TODOs should ideally be of the format: TODO(#issue_number). Have this as a presubmit check?
• ... TBD

Hopefully these will be enough to ease the process of IR standardization #2193