Derive macro doesn't support type aliases for the database

[jfecher]

We should support the code below:

struct MyStorage { ... foo: SingletonStorage<Foo> ... }
struct Foo;
type MyDb = inc_complete::DbHandle<MyStorage>;

#[intermediate(id = 1)]
fn foo(ctx: &incremental::Parse, db: &MyDb) -> i32 {
    0
}

But we get an error when doing so:

error: DbHandle must have a generic type parameter for the storage type
  --> src/foo/mod.rs:84:45
   |
84 | fn foo(ctx: &incremental::Parse, db: &incremental::DbHandle) -> Arc<ParseResult> {
   |                                       ^^^^^^^^^^^^^^^^^^^^^

If we name the alias something else we get an error making it look like the macro is looking for an exact name:

error: second parameter must be &DbHandle<StorageType>
  --> src/foo/mod.rs:86:45
   |
86 | fn foo(ctx: &incremental::Parse, db: &Foo) -> Arc<ParseResult> {
   |                                       ^^^

This is overly restrictive and prevents users from using the derive macros in more situations