switch to new database design

Under this design, *all* databases are a
`DatabaseImpl<U>`, where the `U` implements
`UserData` (you can use `()` if there is none).

Code would default to `&dyn salsa::Database` but
if you want to give access to the userdata, you
can define a custom database trait
`MyDatabase: salsa::Databse` so long as you

* annotate `MyDatabase` trait definition of
  impls of `MyDatabase` with `#[salsa::db]`
* implement `MyDatabase` for `DatabaseImpl<U>`
  where `U` is your userdata (this could be a
  blanket impl, if you don't know the precise
  userdata type).

The `tests/common/mod.rs` shows the pattern.

src/function/execute.rs

@@ -1,8 +1,7 @@
 use std::sync::Arc;
 
 use crate::{
-    local_state::ActiveQueryGuard, runtime::StampedValue, storage::ZalsaDatabase, Cycle, Database,
-    Event, EventKind,
+    local_state::ActiveQueryGuard, runtime::StampedValue, Cycle, Database, Event, EventKind,
 };
 
 use super::{memo::Memo, Configuration, IngredientImpl};