Argument references: Rename the |reference| function to |fun_ref|

We'll need to distinguish function and argument references soon.

This will be mirrored by a similar change in YaST Ruby bindings.
Argument references: Introduce |Y2R::AST::Ruby::Expressions|

This node will be used when calling function with argument references.
Argument references: Calls have a type

This will allow us to perform actions based on function type, like
passing argument references correctly.

The |type| attribute was added to the XML in a recent Ruby bindings
version.
Argument references: Implement correct argument passing semantics

Argument references are now translated into |YCP::ArgRef| instances that
are passed across the call boundary instead of real values. This will
make it possible to assign a value to an argument passed by reference
and pass it out of the called function.
Argument references: Access reference argument values correctly