Add chart functions "x()" and "constant(3)"

Summary:
Depends on D20442. Ref T13279. Add basic support for drawing chart functions that are not based on Facts first-party ETL datasets. Some general goals:

• This might be useful to draw a line like "goal" or "profitability".
• This might be useful to pull data from an external source.
• For composable functions like "add" or "subtract", which are useful in manipulating ETL datasets, these value functions will make testing easier.

Test Plan:
Added a constant(256) function:

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: yelirekim

Maniphest Tasks: T13279

Differential Revision: https://secure.phabricator.com/D20443