Perform subsititutions one after another.

Perform subsititutions one after another.

Previously, multiple substitutions would be done in a single run of
substExpr, but that did not properly handle the case where one of the
to-be-substituted values appears in the to-be-substituted values. For
example, when performing the substitution [(a, b), (b, c)] on the
expression a, we would get b (while we want to get c).