Make the environment info a global field directly.

Also, set its field `exportsNamespace` after the fact with
what we chose to use.

Finally, we also freeze the environment info object when we're
done with initializing Scala.js, to prevent anyone from modifying
it after that.
Fix #1184: Incorporate JSGlobal as a field of environment info.

This gets rid of an IR node and a compiler primitive.

Desugaring still applies a shortcut to simplify
ScalaJS.env["global"]
as
ScalaJS.g
which is quite important, given how much this field is used.