Minimal C++ replacement for rvec

Minimal C++ replacement for rvec

Add a C++ class that acts as rvec, but also supports placement in STL
containers (i.e., is default-constructible, copy-constructible, and
assignable) and can be returned from functions.  There is some ugly
reinterpret_casting required for rvec arrays to make this work, which
requires that the compiler generates compatible layouts for the class
and rvec.  The tests ensure that this is the case, but it could be nice
to check some more exotic compilers as well, unless someone knows the
C/C++ standards well enough to say that this layout is always ensured.

Passing RVec as 'const rvec &' unfortunately also needs an explicit
function call because of issues with cppcheck and MSVC.

Minimal solution to #1017.

Change-Id: I79be58bf7be2a78f787f470044b3b5abf234ec0c