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/config.rs

@@ -8,6 +8,7 @@ use crate::{Connection, Result};
 /// Database Connection Configuration Options
 #[repr(i32)]
 #[allow(non_snake_case, non_camel_case_types)]
+#[non_exhaustive]
 pub enum DbConfig {
     //SQLITE_DBCONFIG_MAINDBNAME = 1000, /* const char* */
     //SQLITE_DBCONFIG_LOOKASIDE = 1001,  /* void* int int */