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/collation.rs

@@ -109,6 +109,7 @@ impl InnerConnection {
         x_coll_needed: fn(&Connection, &str) -> Result<()>,
     ) -> Result<()> {
         use std::mem;
+        #[allow(clippy::needless_return)]
         unsafe extern "C" fn collation_needed_callback(
             arg1: *mut c_void,
             arg2: *mut ffi::sqlite3,