Use MemoryAccessInfo instead of Inst for passing requests between caches and MSS (#144)

As part of #143 use MemoryAccessInfo as the standard transaction type
for making memory requests outside of the core, instead of instructions.

This paves way for adding an instruction cache, as well as prefetching
and more complicated memory subsystems where requests do not always
correspond to a particularly instruction.

Changes are minor, mostly just renaming.

Co-authored-by: Daniel Bone <daniel.bone@imgtec.com>