`finnish` ignores observable-returning functions assigned to properties, variables, params

[JasonWeinzierl]

See initial upstream report: cartant#105

This is likely related to couldReturnType or couldBeType ignoring property signatures, etc.

Possibly related issue: #57 . Since multiple bugs are coming from couldReturnType, a broader re-thinking of that function may be appropriate.

This also impacts no-misused-observables.