Reorganized packm implementation.

Details:
- Reorganized packm variants and structure-aware kernels so that all
  routines for a given pack format (4m, 3m, regular) reside in a single
  file.
- Renamed _blk_var4 to _blk_var2 and generalized so that it will work
  for
  both 4m and 3m, and adjusted 4m/3m _cntl_init() functions accordingly.
- Added a new packm_ker_t function pointer type to
  bli_kernel_type_defs.h
  to facilitate function pointer typecasting in the datatype-specific
  packm_blk_var2() functions.
- Deprecated _blk_var3.
- Fixed a bug in the triangular micro-panel packing facility that
  affected trmm and trmm3 with unit diagonals.