Pack building commands into simple crate

[NikVolf]

instead of

sails/examples/this-that-svc/wasm/build.rs

Lines 4 to 15 in 268e11a

	fn main() {
	gwasm_builder::build();
	let manifest_dir_path = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
	let idl_file_path = manifest_dir_path.join("this-that-svc.idl");
	let idl_file = File::create(idl_file_path).unwrap();
	sails_idlgen::generate_serivce_idl::<ServiceMeta>(idl_file).unwrap();
	}

should be this:

sails_builder::new().generate_idl().build()