Per-benchmark hashes

[jbrockmendel]

IIUC whenever I make any edits in a benchmark suite, the entire suite gets a new hash, so even if a particular benchmark hasn't been changed, it needs to be re-run over old commits to get a continuous history. This makes it expensive to make incremental improvements in a benchmark suite.

It would be nice to be able to mark a benchmark with e.g. @asv.last_changed("2017-04-05") or @asv.hash_value("...") so it gets treated as unchanged. It would be up to the user to ensure these marks are accurate.

[HaoZeke]

This is a good idea, but in practice it would mean a breaking change to the current results format, and would need updates for each project (similar to the change from results v1 to v2).