Change LoadOperation and StoreOperation to use MemRef

[3ulalia]

Currently, LoadOperation and StoreOperation expect the instructions to just be "load" and "store", respectively. This is not possible in current MLIR - indeed, I don't know if it was ever possible. Regardless, since the operations involve MemRef types anyway, this isn't a significant change, and just makes it compliant with modern MLIR.

(fixes the original cause of #42)

Fixes #42.