* cleanup unused feature
* fixup dependency structure
  - as os concept, application of tachyos wouldn't be directly (statically) linked
    to os binary
  - in this reason, api structure is separated from kernel headers
  - 'tch.h' & 'tch_types.h' declare all the data type and interface structure
     used to build application
  - kernel headers define statically linking interface with kernel binary image