Move URI methods from js.package to js.URIUtils.
Remove numeric-related methods and constants in the js package.

The corresponding Scala/Java methods and constants should be
used instead.
The scala.scalajs.js package is not a js.GlobalScope anymore.

It only contained one last native method (as opposed to
primitives), i.e., eval(). Now eval() is implemented in user-space
using the js.Dynamic API.

This transformation allows to implement `undefined` and
`isUndefined` in user-space as well, instead of as primitives.
Merge pull request #1391 from sjrd/rethink-js-package

Fix #1383: Rethink the scala.scalajs.js package