Reconsider local evaluation

[hadley]

Currently partial_eval() locally evaluates calls of the form pkg::bar() (where pkg is not dplyr), and x$y (which will block #158). It's probably worth reconsidering in the light of unquoting as I think this would make the interface more consistent.

At the same time, we could deprecate local() (since it's basically equivalent to !!). remote() needs to stay in case we have override an SQL function with the same name but a different meaning.

[hadley]

Leaving local() as is for symmetry with remote() and to minimise breaking changes.