Do not rely on macros with implicit vars

This commit also provides the benefit that the Label argument
in gen:call/3 is no longer used, as we can access the sender
in the From argument. Therefore, we should be able to change
the Label to something more relevant in the future.
Merge branch 'josevalim/jv-genevent' into maint

* josevalim/jv-genevent:
  Do not rely on macros with implicit vars