This was an oversight before -- the current type implies that
one introduce a synthetic write (and hence a new revision) from
a `Frozen<DB>`! Not cool.
Now, the effect of GC is a "product" of three parameters:
* what values are affected (everything/everything except used)
* are we removing values
* are we removing deps
SweepStrategy::default is now a no-op GC.
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.
