[nomaster] SI-5923 a simple alternative to onInfer

This fix provides implicit macros with an ability to affect type inference
in a more or less sane manner. That's crucial for materialization of
multi-parametric type class instances (e.g. Iso's from shapeless).
Details of the technique can be found in comments.

The changes introduced here are good from the conceptual point of view,
but are sub-standard as far as clarity and cleanliness are concerned.
As the latest advances in implicit macrology show, there's no need for
an elaborate mechanism of keeping track of undetparams in Macros.scala,
therefore we should really clean that stuff out. However, this is 2.10.x,
and I want to minimize the surface of code changes in order to make sure
I don't break anything.

Having said all that, I'd like to note that this commit is only targeted
at 2.10.x. There's no reason for it to be merged into master.