I implemented this in a similar way to the way blotting is implemented in a blot
map vector:
1. I changed this to store (Key, Optional<Value>) pairs.
2. I made it so that once frozen, we can "erase" things from the multimap by
setting all Optional<Value> to none.
3. I changed the range we vend to be an OptionalTransformRange instead of just a
TransformRange so we skip all keys with .none values, meaning that a user will
get the nice behavior that getRange() still works after erasing.
One interesting thing to note is that one /cannot/ erase elements when
initializing the frozen multi-map since we haven't sorted it yet. At first this
seems weird, but it actually fits with the use case of this data structure:
building up state by processing IR in a readonly way and then later working with
it in a worklist like way (and perhaps checking for unhandled cases at the end
of processing).
The nice thing additional thing is that I was able to ensure that the actual
exposed API did not change in terms of how one uses it. I just changed the
underlying iterators/etc.
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