futhark profile: generate more index files

[athas]

Currently futhark profile produces an HTML index file for each dataset. In addition, I would like:

• An index file for each program, with links to the per-dataset index files.
• A top level index file with links to each program index file.

The idea is to make everything navigable via a browser. It is easy to imagine an automated benchmark setup that publishes these profiling reports on the web.