[Rust] Add better support for "crate-per-schema"

Currently, flatc allows using `--rust-module-root-file` and `--gen-all` to generate multiple schemas into a single crate with a top-level `mod.rs`. This is good but makes it really hard to use in many contexts since the best (only?) option to have inter-dependent schemas is to generate everything together into a single crate.

Ideally, we can generate each schema independently of the includes (as each include will be its own generated crate), and link them all at build time.