Refactor moss-deps for (increased) SafeD compliance

[ermo]

Currently, the when attempting to use SafeD, the compiler complains due to the way we use pointers to structs.

For the in-progress moss rewrite (based on the LMDB-based moss-db module), let's update moss-deps to have the following properties:

• Fix @safe on all non-graph bits
• Add global @trusted annotation on digraph code as a short-term workaround
• Add suitable code-style notes from the experience, including thoughts on moving to a class-based approach for SafeD reasons.

[ermo]

Due to unforeseen difficulties related to Dlang's Phobos library not being all the way SafeD yet, we've haxx0red our way around this w/@trusted block-level annotations to avoid blocking on it for the moss rewrite.

Leaving open but unassigned until such a time as Phobos' SafeD coverage is more mature.