Add instead hook

[guillep]

Add support for instead hook.

The instead hook works alone, it is by design combined with before/after.
If you apply the instead hook, you should apply the before and after altogether, in that single hook.
That is, the before/after hooks are ignored even if defined.

This leaves us with four different instrumentation variants.

• before only, then forward to normal method
• after only, forwarding to normal method
• before and after around forwarding to normal method
• instead of normal method

Depends on #38

[jordanmontt]

Great!!!!