Add experimental "unified execution" for scripts and styles.

Split inline script and style evaluation into two functions:
- `eval`: conditionally evaluate the text only if changed
- `exec`: unconditionally execute the text

This matches the behavior of external script and style installation:
- `load`: conditionally install the resource only if changed
- `get`: unconditionally install the resource.

For now, guard the new logic behind a `experimental-execute-unified` config.

Progress on #25.
I'm a bit confused here. Since the stat library is the guard against re-executing duplicate code, what's the reason for holding on to the element here, while it get's deleted by the script.exec and style.exec paths?