Replace instant

[jleibs]

instant has at least one issue affecting us:

• SystemTime::elapsed() is broken sebcrozet/instant#49
  It also hasn't been updated in 18 months.

web-time: https://github.com/daxpedda/web-time appears to achieve the same goals, but is under active development and notably doesn't have the bug that instant had.

There is a PR to do this migration:

• Move from instant -> web_time #2093

We hit a few issues during testing, but they may have been unrelated. Opening this issue so we don't forget to come back to it.