R1:954831f5336b

Separate chart functions into a class tree

Summary:
Depends on D20440. Ref T13279. Create a class to represent a chartable function: something we can get some data points out of.

Then, make the chart chart two functions.

For now, the only supported function is "fact(key)", which pulls data from the Facts ETL pipeline, identified by "key", and takes no other arguments.

In future changes, I plan to support things like "fact(tasks.open.project, PHID-PROJ-xyz)", "constant(1000)" (e.g. to draw a goal line)…