GitLab

Currently, the following API-usage is straight-forward and clean but
unfortunately wrong and dangerous:

`for (const auto heh: mesh.face(fh).halfedges()) { ... }`

This is because the temporary `Face` returned by `face()` reaches the
end of its lifetime, but `halfedges()` returned a reference to a vector that
lives inside that object. Reference lifetime extension does not apply
here.

This change splits behaviour so that in rvalue context (such as
temporaries), we move out the vector instead of returning a reference to
it.

These constructors were introduced in 26e2fe64 to implement clone(),
but their signature might suggest uses that would result in memory corruption.

Fixes #5.

MSVC does not define __cplusplus as >=201103 yet.
An older version of MSVC is probably sufficient, but I can't test that.