Add export CPP for MatMul operator:
- supports scalar/1D inputs
- provision for activation as done in elemwise operators
- new kernel matmul.hpp provided
- TODO: as of now do not support >2D inputs/broadcasts, see below

This change does no include the support for >2D inputs.
It would require managing multi broadcast
which is also not supported for CPP export of elemwise operators.
This support is delayed until a further change factorizing
multi-broadcast on elemwise/matmul and after availability
of a more comprehensive unit tests suite.