Use a different syntax for use

* Remove strings as targets-for-`use` and instead always use from
  identifiers. This pushes resolution and such up a layer where it's
  going to be happening anyway between packages.

* Move the module that's being used from to the start of `use` to better
  assist with auto-completion in IDEs possibly in the future. This way
  we as humans will type `use foo.bar.{` and an IDE should have enough
  context by that point to auto-complete what to use.

* Add a `pkg` and `self` path anchor which paths start from to indicate
  that they're rooted in the current package or current document.

* Continue to use `.` as a separator between items in a `use` statement.

Author: alexcrichton