New feature request: Output macro source at invocation point when in --display mode.

One of the things that folks don't understand well (particularly new Haxers) is how macros work, and what their macros are actually doing. It would be nice for IDEs to be able to pop up a window showing the (source) results of their invocation. To do so, the compiler can probably do the following:

(According to @nadako:) ... - when typing a macro call, check if we're in our special display mode and the requested position is within the call pos and return the stringified expr