Silent some clippy warnings (#924)

* allow(clippy::upper_case_acronyms) for rust enum entries that match
  SQLite constants.
* allow(clippy::needless_return) for collation_needed_callback until we
  find a way to propagate the error.

src/vtab/mod.rs

@@ -261,6 +261,7 @@ pub trait CreateVTab<'vtab>: VTab<'vtab> {
 /// See [Virtual Table Constraint Operator Codes](https://sqlite.org/c3ref/c_index_constraint_eq.html) for details.
 #[derive(Debug, PartialEq)]
 #[allow(non_snake_case, non_camel_case_types, missing_docs)]
+#[allow(clippy::upper_case_acronyms)]
 pub enum IndexConstraintOp {
     SQLITE_INDEX_CONSTRAINT_EQ,
     SQLITE_INDEX_CONSTRAINT_GT,