make spec infra more ergonomic

Signed-off-by: Onur Özkan <work@onurozkan.dev>

kaptan-spec/tests/ui/fail_missing_argument.rs

@@ -1,6 +1,5 @@
-use kaptan_spec::spec;
+use kaptan_spec::no_spec;
 
-#[spec]
-struct MissingArgument;
+no_spec!();
 
 fn main() {}