Feature: Send endpoint for attributes

• enable send endpoint targeting attributes, i.e., use normal attributes to send their value whenever invalidated
• this would then probably the default for an endpoint type in the DSL, but it is unchecked (because aspects are neither passed as arguments nor parsed). the error would be reported by jastadd earliest
  • could be mitigated by alternative syntax using () after name of attribute, or using with unchecked, or as attribute

DSL:

send A.value;  // would work
send A.nonexistentAttribute;  // error in JastAdd, or in javac

send A.value();  // possible alternate syntax to denote an (unchecked) attribute,
                 // other endpoint types would be checked as usual