TL;DR; Add a first-class multimap to x/sync

Why do we need a specific primitive for multimap?

1. standardization - Libraries can more easily inter-operate if there is a standard multimap type. Rust and C++ offer standard multimap implementations, demonstrating the need/demand for this type.
2. correctness - Implementation of multimap is non-trivial. Providing a standard implementation reduces the risk of bugs introduced by poor multimap implementations.
3. performance - A multimap structure using existing primitives (eg. maps and slices) is likely to be sub-optimal because of redundant locking. Creating a first-class multimap type allows for a more efficient implementation using unsafe primitives.

Reference implementation:

The following is a reference implementation using existing (map + mutex) primitives:

https://go-review.googlesource.com/c/sync/+/517955