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.
The only code path where the element is deleted immediately is `script.exec -> resource.exec`.  For `script.eval -> resource.eval -> resource.exec`, `style.exec -> resource.exec`, and `style.eval -> resource.eval -> resource.exec`, the element is retained.  For styles, the element must be in the DOM to apply, and for scripts, I think the presence in the DOM provides a nice indication of what's being managed and is currently active.