Hide zlib completly from user. This way we do not need to publish zlib.h
or zconfig.h.

As we now have two different pointer initalizing in constructor I choose
to use unique pointers so it is more obvious those are safe. I also
choose to use make_unique which default initalize z_stream_t. This makes
code more readable as we do not need to specifie every field of
z_stream_t. It really should not matter much if we initialize couple
field for nothing. If does we should add comment about that. Still
keeping _buffer without inializing as it is quite big.

Hide expat completly from user. This way we do not need to publish
expat.h or expat_external.h.

I move also headers to orignal locations so diff is smaller compared to
original.