Specify callable object closurization

[eernstg]

This PR adds a new case to the set of possible 'property extraction' expressions: Implicit property extraction. It occurs where an expression e has a static type as a callable object, and the context type is a function type. In this case we desugar e into e.call. Note that this will also serve as the first step in a generic function instantiation, because it is still true that the context type is a function type also in the case where e.call isn't assignable to that context type (but we can fix it by means of an implicit instantiation).

Smaller corrections:

The subsubsection about generic function instantiation was turned into a subsection: It used to be the case that a generic function instantiation would necessarily include a function closurization as a sub-action. This is no longer true, we can perform the generic instantiation on an existing function object. This means that the generic instantiation material should no longer be a subsection of 'Function Closurization', it's a stand-alone feature.

Subtitle uses version 2.15-dev, not 2.13-dev.

Note that this change is not expected to give rise to any implementation efforts. The following works today already:

class A { call() {}}
void f(void Function() g) => print(g);
void main() => f(A());

[github-actions]

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