Normalize Flat subexpressions

Currently, normalization on flat expressions only attempts to match the pattern to the full expression, not subexpressions. Thus, given the rule `f(x) * g(x) => x`, the term `f(1) * g(1)` will return `1`, but the term f(1) * g(1) * h()` will not normalize.

Locate the normalization entry point and the existing matching path for flat expressions. Compare the two examples in the issue, then verify that a matching flat subexpression is normalized even when other terms remain in the expression.

Written by the indexing model from the issue text.