After discussion (starting there:
https://github.com/gbdev/rgbds/pull/594#issuecomment-706437458
), it was decided that plain nested macros should not be
allowed.
Since #590 is fixed, EQUS can be used as a workaround;
multiline strings (#589) will make that easier on the
user when implemented.
Fixes#588, supersedes and closes#594.
Additionally, closes#388.
