Implement desugaring for the StaticValues extension.

    static f
==>
    StaticRef $
	  GlobalName "package id of f"
	             "package-installed suffix"
	             "module of f"
				 "name of f"
Make sure identifiers given to static are exported.
Add HsStatic case to coverage code.
Fix T4437 so it succeeds with the StaticValues extension.
Implement tests of the StaticValues extension.
Allow static to accept any closed expression.

Expressions are floated to top-level bindings with fresh names.