Quickest POC I could create to get some potentially cyclic queries to
not panic and instead return a result I could act on. (gluon's module
importing need to error on cycles).
```
// Causes `db.query()` to actually return `Result<V, CycleError>`
fn query(&self, key: K, key2: K2) -> V;
```
A proper implementation of this would likely return
`Result<V, CycleError<(K, K2)>>` or maybe larger changes are needed.
cc #6
Switch to a procedural implementation of the `query_group!` macro,
residing in the `components/salsa_macros` subcrate.
Allow the user to override the invoked function via `salsa::invoke(...)`
and the name of the generated query type via `salsa::query_type(...)`.
In all tests, replace the `salsa::query_group! { ... }` invocations with
the new attribute-style `#[salsa::query_group]` macro, and change them
to the new naming scheme for query types (`...Query`).
Update README, examples, and documentation.
