Be more careful about final top-level substitution.

Be more careful about final top-level substitution.

Substituting x==log(x) in exp(x) erroneously returned log(x) because of a
final subst(x==log(x)) after having eval'ed exp(log(x)) -> x. This final
substitution is wrong in the general case. On the other hand, the intent
is to syntactically substitute functions of a given kind etc. This patch
suppresses the final top-level substitution unless the intermediate result
is a container.

Thanks to Burcin Erocal for reporting this bug (originally described by
Kees van Schaik on sage-support@googlegroops.com).