Lint: multiple factory definitions that make the same object

[mike-burns]

Idea for a lint: complain if two factory definitions create the same object. This is a goal towards these ideals:

• each factory definition is the minimum required to make the object
• anything additional is added via traits

We'll need to consider factories that make e.g. Hash, String, Array -- either by letting the user turn off the linter for them, or by recognizing them as OK.