Move C++ "Constructors" into obscurity

This makes macros available for inline constructor definitions, rather
than having them be displayed prominently in the public header.

This also changes the name of cxxwrap.h to _cxxwrap.h to further
indicate the 'do not use this' aspect of that file.

Change-Id: Ia3784f66f7e1dce065205f1a02786f3e0a0a0083
Reviewed-on: http://review.couchbase.org/42129
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>