R1:70c643c685f8

Simplify implementation of "pure" Chart functions

Summary:
Depends on D20445. Ref T13279. I'm not sure what the class tree of functions actually looks like, and I suspect it isn't really a tree, so I'm hesitant to start subclassing. Instead, try adding some `isSomethingSomething()` methods.

We have some different types of functions:

# Some functions can be evaluated anywhere, like "constant(3)", which always evaluates to 3.
# Some functions can't be evaluated anywhere, but have values everywhere in…