3182 - primitives for manipulating the file system

I don't know why this took so long to gel. I just needed to copy my
approach to screen management:

1. primitives layer (C++): simple, non-testable, non-safe operations.
2. wrappers layer (Mu): wrap operations with dependency-injected
versions that can take a fake file system.
3. scenario layer (C++): implement assume-filesystem that constructs a
fake file system.
4. scenario test layer (Mu): test out assume-filesystem in a test.

This commit implements step 1.

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