Hardcode UniqueID in streams keys

[keeshux]

Now that Foundation is ubiquitous, we can use UniqueID/UUID directly. Fixes a weird runtime crash in a missing Hashable witness table (OID → RandomlyInitialized → Hashable).

[keeshux]

Something similar had happened when linking to libpartout.a statically, where Hashable implementation metadata were lost in the linking process. There was a runtime crash on dict[key] = value because the Swift runtime was not able to find the Hashable implementation of the key type (LoggingCategory IIRC).