Speed up NLL with `HybridIdxSetBuf`.

`HybridIdxSetBuf` is a sparse-when-small but dense-when-large index set
that is very efficient for sets that (a) have few elements, (b) have
large `universe_size` values, and (c) are cleared frequently. Which
makes it perfect for the `gen_set` and `kill_set` sets used by the new
borrow checker.

This patch reduces the execution time of the five slowest NLL benchmarks
by 55%, 21%, 16%, 10% and 9%. It also reduces the max-rss of three
benchmarks by 53%, 33%, and 9%.

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