TreapMap union and intersection

[ericeil]

TreapMap.merge is a very flexible way to merge two TreapMaps, but it has a couple of drawbacks:

1. It always visits every entry in both maps
2. It doesn't allow null values to be added to the result map (because merge treats null as meaning "remove this entry")

The first issue can be a major performance problem if you're merging two large maps with a relatively small key intersection, and you only need to apply custom merge logic to the keys in the intersection. This turns out to be very common.

To address this, we add new methods to TreapMap:

    public fun union(
        m: Map<K, V>,
        merger: (K, V, V) -> V
    ): TreapMap<K, V>

    public fun intersect(
        m: Map<K, V>,
        merger: (K, V, V) -> V
    ): TreapMap<K, V>

These apply the merger function only to the intersection of the two maps' keys, and don't permit merger to discard entries. union preserves all non-intersecting entries from both maps, while intersect discards them.

We also add parallel versions of both functions, following the example of merge/parallelMerge.

See https://github.com/Certora/EVMVerifier/pull/6819 for an example of where this can be used to get some significant performance benefits.

[jtoman]

good modulo a spelling error

[jtoman]

"temporary"

[ericeil]

thanks