Implement calling_env()

[lionel-]

This implements a helper function that makes it easy to retrieve the original environment that called a pipe chain. This will help magrittr-aware tools to reinforce the idiom that a pipe chain is one single function entity.

Another way of implementing this would be to climb the calling stack dynamically, looking for magrittr's anchors (_fseq, _function_list, etc). This way calling_env() would work when called in functions embedded deeply in pipe components. If we start from n = 5, there would be no cost when calling_env() is called from the top level of a pipe component. But I suspect it may get very costly if called outside of a pipe chain and the calling stack is large.

[hadley]

Fixes #171

[lionel-]

We should probably use a different approach, like a marker environment installed in the data mask / exec-env. Ideally magrittr wouldn't use a child environment at all.

[smbache]

This should be so much simpler in the suggested approach where everything is evaluated in that one function

[lionel-]

I think it might make sense to evaluate magrittr expressions directly in the calling environment, I'll reply to the thread with Hadley soon!