Previously they were defined in manifold.h but their implementations
were in the internal directory and to prevent circular dependencies
the implementation headers were pushed to the bottom of manifold.h
This started out as one header and has become progressively worse
as more manifolds are templated.
This change moves the two manifolds into their own headers which
also contain their implementations.
Change-Id: I671da0279a47cd2ff1f52c69a1d159426f55bd80
Sameer Agarwal
eadfead69d