On deinit warning, also log location where the PromiseSource was created

[mac-cain13]

This would be very valuable when trying to fix the error, maybe only capture/print this info in debug mode since it is a bit sensitive info.

The magic constants like __FUNCTION__ should make this possible.

[tomlokhorst]

This is indeed very useful.

We should also think about how to handle PromiseSources created by combinators like map and flatMap.

Promises often go through a stack combinators, like: somePromise.map(...).mapError(...).flatMapResult(...).
We probably want to print the entire stack, since either end isn't sufficient information. Especially when a promise has been passed around a bunch.

[mac-cain13]

Yeah, agreed. I think a promise based on another PromiseSource should print the location of the origional source. Or if possible the whole stacktrace indeed, but I don't know how much value that adds.

[tomlokhorst]

This is being worked on in #19