Add an annotation asserting that a value is a return value

[chrispyles]

Add an annotation that is similar to a value annotation but which specifically asserts that a value was returned by a function (that is, was the value of arg when the trace function was called for the return event).

Consider matrix exponentiation A^p when p == 1: the matrix A always shows up in the memory footprint as long as it's used inside the function body, but the return value of the function is not necessarily A. This may also help design references for problems that suffer from the issues described in #62.