Support @ as a new operation via BinaryOperation for accessing fields

This would allow tree structures to be used in an XPath kinda way. e.g. if you had a tree of nodes you could do

node.@x

to refer to an attribute rather than

node.x

which can refer to children.

Then Groovy would have similar operations to XPath

Similarly @x could be used too.

This would also allow fields to be explicitly referenced as opposed to properties. e.g. something.@foo if something is a friend/in same package etc.