Remove R

[smarr]

This PR is the final step that removes R from ReBenchDB. It replaces the old compare view with the new one, and removes all R files and dependencies.

The performance is as hoped, much better. Rendering a report takes now:

small    314ms (-93%, was  4431ms)
medium   562ms (-92%, was  7216ms)
large   2409ms (-86%, was 17650ms)
full    6927ms (-93%, was 96179ms)

See https://rebench.dev/ReBenchDB/compare/6826fcfb0da82a177bcc7c5f046bb08424d5cd23..02aa1af78efc68b919a161e334a8cf9e4eb56765

(these numbers are taken before modifying the benchmark to not write the HTML file, which had no major impact but simplifies the benchmark)

This fixes also the following things:

• add caching #155
• remove mustache template engine #144