This makes checking of variables more strict: a variable can only
be used in the right -hand side of an assignment if it has been
assigned by an unconditional predecessor or if it has been matched
by a parent.
Things that wouldn't work anymore:
REF=X* {
FN=*V { X=Y }
Z=$X /* we can't be sure X=Y was taken */
}
FN=*R {
X=$REF /* no previous use of REF */
}
The ${foo#unit} syntax didn't really make sense because it created
a large number of potential error conditions on the assignment side
and didn't help with finding compatible fields.
With all this moved to the match side, an invalid syntax simply causes
a mismatch.
