Opt-in to another clippy lint

Cargo.toml

@@ -495,6 +495,7 @@ ref_option_ref = "warn"
 rest_pat_in_fully_bound_structs = "warn"
 same_functions_in_if_condition = "warn"
 semicolon_if_nothing_returned = "warn"
+set_contains_or_insert = "warn"
 should_panic_without_expect = "warn"
 single_char_pattern = "warn"
 single_match_else = "warn"