What about backing cid.Set by a sorted slice rather than a map?

It's pretty common when using `cid.Set` to want them to remain sorted. For example, if you're going to compare two sets, you need them to be sorted. If you're going to compute a hash representing the set, you need to do it over a sorted set, otherwise two hashes for equal sets won't be equal.

We could introduce a new `cid.SortedSet` for this use case, but I think it is worth considering making `cid.Set` always sorted and backing it by a sorted slice rather than a map.

I can't recall the exact details but I think insertion and lookup in a sorted slice in Go is negligibly different performance than into a map. More importantly, with a slice, you have the option of using a stack allocated array as the backing buffer, eliminating a heap allocation, reducing gc pressure.