* Custom deep learning layers support

* Stack custom deep learning layers

via GCC/Clang __POPCNT__ define

improved performance of v_load_deinterleave(8uC3) & v_store_interleave(8uC3) intrinsics when using SSSE3 instructions.

To dump contents of the last OpenCV error

v_select 'mask' is restricted to these values only: 0 or ~0 (0xff/0xffff/etc)
mask in accuracy test is updated.

* Issue 11242 intrinsics v_extract, v_rotate improvement, branch 3.4, without C++11 (remove type restrictions for SSE2, use PALIGNR on SSSE3, compile to no-op when imm is 0 or nlanes).

* fix whitespace

* Fix #11242 (NEON intrinsics v_rotate...) branch 3.4
Separate macro expansion OPENCV_HAL_IMPL_NEON_SHIFT_OP for bitwise shifts for integers, from macro expansion OPENCV_HAL_IMPL_NEON_ROTATE for lane rotations. Bitwise shifts do not apply to floats, but lane-rotations can apply to both.

* fix whitespace

* Fix #11242 compile error (VSX intrinsics v_rotate(a)) branch 3.4 no-c++11

* Fix CV_Asserts with negation of strings

{!"string"} causes some compilers to throw a warning.

The value of the string is not that important -- it's only for printing
the assertion message.

Replace these calls with:

  CV_Error(Error::StsError, "string")

to suppress the warning.

* remove unnecessary 'break' after CV_Error()

Error messages are available via exceptions