refactor(core): Improve ergonomics of managing ASCII strings (#18498)

This is a follow-on to the earlier work in reducing string copies,
mainly focused on ensuring that ASCII strings are easy to provide to the
JS runtime.

While we are replacing a 16-byte reference in a number of places with a
24-byte structure (measured via `std::mem::size_of`), the reduction in
copies wins out over the additional size of the arguments passed into
functions.

Benchmarking shows approximately the same if not slightly less wallclock
time/instructions retired, but I believe this continues to open up
further refactoring opportunities.

cli/js.rs

@@ -22,7 +22,7 @@ mod tests {
       ..Default::default()
     });
     js_runtime
-      .execute_script(
+      .execute_script_static(
         "<anon>",
         r#"
       if (!(bootstrap.mainRuntime && bootstrap.workerRuntime)) {