Define an API for the transformational intrinsic function MATMUL,
implement it, and add some basic unit tests. The large number of
possible argument type combinations are covered by a set of
generalized templates that are instantiated for each valid
pair of possible argument types.
Places where BLAS-2/3 routines could be called for acceleration
are marked with TODOs. Handling for other special cases (e.g.,
known-shape 3x3 matrices and vectors) are deferred.
Some minor tweaks were made to the recent related implementation
of DOT_PRODUCT to reflect lessons learned.