Add Arguments, PositionalArgumens

Called -ArgumentList in the RedHat AST.
Add tests for Arguments, PositionalArguments
Primary: fix doc link to Operation

I had originally planned to call Operation OperatorExpression, and only
use the -Operation suffix for the subclasses.
BaseExpression test: extract construct() function
Add Invocation (RedHat’s InvocationExpression)
Add tests for Invocation backend conversion