Fix three bugs in Faust-vector matrix.

- The first dates of a recent optimization (which avoids copies) and occurring only in the transpose case.
- The second is concerning only matlab and was introduced when the matfaust was moved to the interleaved complex API (the output buffer of the mul. wasn't initialized).
- The third concerned also only matlab, precisely Faust.mtimes_trans function and occurred only in the transpose case too (F'*x wasn't bugged, only mtimes_trans).

Also adding TransformHelperGen::transposed2char().