Write a wrapper API for Fathom in Firefox

[biancadanforth]

Many of Fathom's prospective customers are interested in using Fathom for their Firefox features. In addition to landing the Fathom framework in Firefox, we should think about what kinds of additional functionality may be required or desirable. Some ideas include:

• Execution framework:
  • Based on the results thus far of Make fast enough for untriggered use #91 , it's looking like we will need to execute Fathom concurrently in the main thread.
• Monitoring:
  • For ruleset confidence:
    • Use telemetry to record Fathom's confidence [0,1] in its results over time relative to a confidence cut-off -- is it deteriorating? This would be an indication that a ruleset may need updating.
    • We may also want to record the locale for the page, as we don’t know currently if a single ruleset/set of coefficients/biases will work across all locales or not.
  • For performance:
    • Use telemetry to record how long (wall clock time) it takes a ruleset to run on a page. Depending on the application, this may also suggest how timely Fathom's results are (e.g. identifying login forms for the Password Manager)
    • Use telemetry to record any Fathom-related jank. This can tell us how much Fathom is contributing to the unresponsiveness of the page especially at critical times like page load. Covariates here could include hardware information.