* Refactor TraceState
* Update tracecontext propagator to use new TraceState
* Add TraceStateFromKeyValues to oteltest
* Use oteltest to test TraceState
* Replace IsEmpty with Len for TraceState
* Replace ParseTraceState with ParseTraceStateString
* Clean up naming
* Add immutability test for TraceState
* Add changes to changelog
* Fixes
* Document argument type change in changelog
* Address feedback
Remove circularity of TestTraceStateLen.
