Generic Associated Types in Database, replacing HasValueRef, HasArguments, HasStatement (#2973)

* HasValueRef, HasArguments, HasStatement -> Database GATs

replace the associated types from the generic traits
`HasValueRef<'r>`, `HasArguments<'q>` and `HasStatement<'q>`
with generic associated types in `Database`

* fixup after rebase

---------

Co-authored-by: Austin Bonander <austin.bonander@gmail.com>

sqlx-core/src/testing/fixtures.rs

@@ -1,6 +1,6 @@
 //! TODO: automatic test fixture capture
 
-use crate::database::{Database, HasArguments};
+use crate::database::Database;
 
 use crate::query_builder::QueryBuilder;
 
@@ -111,7 +111,7 @@ impl<DB: Database> FixtureSnapshot<DB> {
 /// which appends to an internal string.
 impl<DB: Database> ToString for Fixture<DB>
 where
-    for<'a> <DB as HasArguments<'a>>::Arguments: Default,
+    for<'a> <DB as Database>::Arguments<'a>: Default,
 {
     fn to_string(&self) -> String {
         let mut query = QueryBuilder::<DB>::new("");