Rename |_element| to |_expression| in AST
Rename |_element| to |_expression| in AST

All nodes which have one subexpression store it in the |_expression| property
now.

mcanthony/pegjs,jlturner/pegjs,crguezl/pegjs,Mingun/pegjs,wwall/pegjs-fn,pegjs/pegjs,luisvt/pegjs,Sciumo/pegjs,luisvt/pegjs,pegjs/pegjs,mcanthony/pegjs,crguezl/pegjs,GerHobbelt/pegjs,Mingun/pegjs,jlturner/pegjs,wwall/pegjs-fn,Sciumo/pegjs,GerHobbelt/pegjs