Refactor MUL instruction

This instruction had a bit of a case of 'boolean blindness' code smell,
where the mul operation was represented as a triple of booleans. This
commit refactors the implemention to use a struct with named fields for
high, signed_rs1, and signed_rs2.

The C_MUL instruction in Zcb also needs to be changed appropriately

The mul_op struct was added in riscv_types

While there do some housekeeping w.r.t the comment about a workaround for
Sail < 0.15.1, as this is no longer needed.