Generalize integrand f to any callable

[mikeingold]

For some integral(f, geometry...), our API currently requires f::Function. However, the only real requirement for f internally is that it must expose a method f(::Point), which can be fulfilled by other callables. For example:

struct Widget
    ...
end

(w::Widget)(p::Point) = ...

w = Widget(...)
geometry = ...

integral(w, geometry)

Enabling this would require

• a sweep of all integral and _integral methods to remove type params from f
• updating documentation, e.g. mentioning this fact in the README
• evaluating whether this causes any dispatch issues
• adding a test that specifies a callable non-Function for f

[JoshuaLampert]

Why is this breaking?

[mikeingold]

Why is this breaking?

I don’t think it’s strictly breaking, but I added the label as a marker to remember that I’d like to complete this one prior to this upcoming breaking v0.16 release.