[multimapcache] Add an efficient CRTP based write-once multimap cache that can be small.

[gottesmm]

The properties of this multimap cache are:

1. Values are stored (inline if Small) in a Vector and our map internally maps
   keys to (start, length) of slices of the Vector. This is done instead of
   storing arrays refs to ensure that if our array goes from small -> large, we
   do not have stale pointers.

2. Values are only allowed to be inserted all at once. This is ok, since this is
   a cache.

3. One is not storing individual small vectors in a map (or state storing
   SmallVectors). This can inadvertantly add up to using a lot of memory and is
   not needed for homogenous data.

[gottesmm]

@swift-ci test

[gottesmm]

@swift-ci test asan

[gottesmm]

@swift-ci asan test

[gottesmm]

@swift-ci smoke test

[gottesmm]

@swift-ci smoke test

[gottesmm]

@swift-ci smoke test

[atrick]

I don't think CRTP buys anything here but it does make the code confusing both here and on the client side.
From the client's perspective this is neither a multi-map nor has any additional caching support beyond a normal map. It's just an array map that happens to be implemented as a flat array. I'd call it something like FlatArrayMap and keep the design simple and self contained.

[EDIT] I read the code more carefully. It is caching a None result, so it is definitely appropriate to call it a cache. Calling it a multi-map is a serious conceptual problem because the client associates a complete array with a single instance of the key. It's critical to understand that the array is all-or-nothing, signifying a complete vs. incomplete analysis!