Implement related_decls tool

[fw-immunant]

Fixes #1563.

This builds and works, with the following caveats:

• Uses of an item are returned as source locations; these need to be resolved to their immediately containing item.
• Output should be formatted as JSON.
• Function signatures are not printed cleanly; we dump R-A's internal representation instead of generating Rust source for types.
• We might want to select only some outputs (uses/used items/fn signature), and we might want to accept multiple paths at once as input. This is up to what c2rust-postprocess and the rest of the CRISP loop needs.
• No automated tests as of yet.