Merge branch 'develop'.
Merge branch 'develop'.
Merge branch 'develop'.
Merge branch 'develop'.
Remove the `_pinkerton` function.

Make `\Phuedx\Pinkerton\Pinkerton::spyOn` and `::stopSpyingOn` static.
Remove `\Phuedx\Pinkerton\Pinkerton` private helper methods.

Initially, `\Phuedx\Pinkerton\Pinkerton::spyOn` was intended to spy on
both functions and methods. This was made explicit by - prematurely -
refactoring the body of the `::spyOn` method into the `::spyOnFunction`
method. Since Pinkerton only focusses on spying on functions, this
refactoring has been undone.