Previosly, panicking query would remove in-progress memos from the
table.
However, we use panic for cancellation, so it's reasonable to **not**
remove the old result after panic. This is also known as string
exception safety guarantee: not only database is in *some* consistent
state after a panic, it is in the same state it was before the panic!
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.
