Add a clippy lint

Cargo.toml

@@ -498,6 +498,7 @@ rest_pat_in_fully_bound_structs = "warn"
 same_functions_in_if_condition = "warn"
 semicolon_if_nothing_returned = "warn"
 should_panic_without_expect = "warn"
+single_char_pattern = "warn"
 single_match_else = "warn"
 str_split_at_newline = "warn"
 str_to_string = "warn"