Stacks 2.1 cost benchmarks: to-consensus-buff and from-consensus-buff

[kantai]

We need to add cost functions and benchmarks for to-consensus-buff and from-consensus-buff.

I believe that both of these methods are linear: to-consensus-buff in the size of its argument, and from-consensus-buff in the size of its second argument.

[jcnelson]

For from-consensus-buff, the runtime is O(n log n) due to tuple keys needing to be stored in sorted order in a BTreeMap.

[jcnelson]

Closing in favor of #3115

[blockstack-devops]

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