Add #[non_exhaustive] to enums that might get new variants. (#673)

This just using them in patterns without a catchall. I left things alone
that seem very unlikely to change (`Value`, `ValueRef`, `DatabaseName`,
etc...). This might help reduce the number of breaking changes we need
(rusqlite is still pre-1.0 so it doesn't really matter that much, but
breaking changes complicate the story around when we can cut releases).

src/statement.rs

@@ -822,6 +822,7 @@ impl Statement<'_> {
 /// may not be available.
 #[repr(i32)]
 #[derive(Clone, Copy, PartialEq, Eq)]
+#[non_exhaustive]
 pub enum StatementStatus {
     /// Equivalent to SQLITE_STMTSTATUS_FULLSCAN_STEP
     FullscanStep = 1,