Mock for functions

[tulzke]

We need the ability to create mocks for functions, not just for interfaces.
It can be usually in cases, where we use callback functions.
Example:

type RunFunc func(ctx context.Context) error

type Runner struct {
  // ...
  run RunFunc // cannot mock this function now
}

I don't want to use interface here, because it imposes a restriction on the name of the method. All I need is a contract match.

Feature request from old gomock: golang/mock#519

Same feature is available in mockery now