swift/ref: Implement decodeId in CGO layer
swift/ref: Implement decodeId in CGO layer

Instead of replicating the Go logic in Swift (and Java), wrap the
existing util.DecodeId in a CGO-accessible method.

Also renames Id.swift to Identifier.swift.

Closes vanadium/issues#1400

MultiPart: 1/2
Change-Id: I1877c609c49043fc9f20121d91de335a0a976bf3

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref