Add an API to manipulate observers at runtime

[bwoebi]

This covers the use case of being able to dynamically trace and untrace functions. E.g. functions, which shall only be observed in a specific context.

It is not a very flexible API (it essentially does a lookup for the old observer (which needs to be specified), then removes it, nor does it allow adding multiple observers at once, unless your extension registers multiple observer init handlers), but it should be good enough for basic use cases.

[krakjoe]

This looks like a good start, pretty simple to understand (and so debug).

While there may be more complex use cases requiring a more complex API, they are probably best implemented by the owner of the those cases.