Add unit testing for FP16 IGEMM kernels

This change adds unit testing for the FP16 IGEMM kernels. The code is
written in a type agnostic manner, as to easily allow testing for other
data types with very low effort. This required the addition of
non-templated `read`/`write` functions, as to allow runtime-generic
access.

The big change is the `imatmul_test.cpp`, which is the main driver of
the FP16 IGEMM testing.

The generated test data is cached, which introduced the need for hashing
functionality in several places throughout the testing code.

Clamp testing is parameterized, which introduced changes needed in the
clamping code.

Finally, test description generated has been updated to allow re-use of
existing type printing

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Reviewed-by: Jakub Sujak <jakub.sujak@arm.com>
Approved-by: Jakub Sujak <jakub.sujak@arm.com>