Rework rule-based labeling function mechanism

[phurwicz]

We need to address the interactivity of rules, i.e. the ability to create them dynamically without significant explorer overhead.

• for example, adding just one rule for crosscheck should not require invoking a whole recipe again.
• there's also no point to stick the naming to snorkel-like, since we don't go any further than applying rules.
  • consider dropping the snorkel dependency altogether because the package is no longer actively maintained.
  • this also drops the assumption that "rules only deal with text data".

Below let's list some options in increasing order of flexibility(?) and decreasing order of feasibility.

Option 1: (most feasible) in-Jupyter-only solution

Give a reference (of the list of rules) to the crosscheck recipe, which has a callback that looks up the reference and runs latest rules. Because this is in Jupyter, python kernel and coding area for the rules are easily available.

Option N: (most flexible) in-any-server solution

Use some widgets to provide the mechanism of adding/removing rules. This will require a text input, a toggle list of current rules, and a remove button. The tricky part will be dynamically turning the text input safely into rules and keeping track of them.

[phurwicz]

Option 1 is fulfilled by https://github.com/phurwicz/hover/commit/9e642a557b41691514ec72429a51c162de6dcc8b