Use bottom_up in fuzzer template substitution

[bbyalcinkaya]

Replaces top_down with bottom_up to avoid redundant traversals in replacement terms, improving performance for large terms. While replacement terms are usually not large, this change enables an optimization in komet that results in a 3x speedup.

The optimization in komet works by embedding large configuration cells without variables directly into a replacement term. This prevents the substitution algorithm from needlessly traversing these large terms, significantly reducing overhead.

• Optimize fuzzer by avoiding traversal of the whole configuration in term substitution komet#69

[gtrepta]

LGTM, good catch. I recall picking top_down over bottom_up for performance reasons, I don't remember how I decided that and admittedly didn't test it.