Portage/Dependency/Normalize.hs: implement 'lift_context' normalization

Most advanced optimization to date.
It float outs common subdependencies in outer context:

    || ( ( foo bar )
         ( foo baz ) )

gets translated to

    foo
    || ( bar
         baz )

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>