Parse use-site variance

Also, extract parse functions for Variance, TypeArgument, TypeArguments.
Add tests for use-site variance

Also, new test naming guidelines: singular, not plural.
The motivation for this is the conflict between typeArgument() and
typeArguments() (typeArguments() and typeArgumentses()?), even though in
the end we can’t have a test for individual typeArgument()s, because
there’s no ceylon.ast.redhat compile function for them.
Format source code a bit