Summary

As currently written, a symbolic LC is created on every operation. We should expect that most Symbolic LC's are small, and probably should try to keep things on stack via small vec optimizations, perhaps using https://github.com/servo/rust-smallvec.

We definitely shouldn't switch without a more detailed benchmark suite (getting average symbolic LC size, seeing how much of the time is spent in mallocs, seeing if things actually get faster etc.)

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