chore: split out simp_mem to Arm/Memory/Common [1/?] (#230)

### Description:

This will be used in the next PR to build `mem_omega`, which will be
a finishing tactic that will use the full power of `omega`. `simp_mem`
will be restricted in power to only perform rewriting, allowing us to
better control `omega`.

### Testing:

conformance succeeds 

### License:

By submitting this pull request, I confirm that my contribution is
made under the terms of the Apache 2.0 license.

Co-authored-by: Shilpi Goel <[email protected]>