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/vtab/mod.rs

@@ -247,6 +247,7 @@ pub trait CreateVTab: VTab {
 /// `feature = "vtab"` Index constraint operator.
 #[derive(Debug, PartialEq)]
 #[allow(non_snake_case, non_camel_case_types)]
+#[non_exhaustive]
 pub enum IndexConstraintOp {
     SQLITE_INDEX_CONSTRAINT_EQ,
     SQLITE_INDEX_CONSTRAINT_GT,