Initial SpanContext definition (#102)
Initial SpanContext definition (#102)

* Initial SpanContext definition

* W3C TraceContext improvements

* link to official version of spec
* corrected casing on Tracestate
* moved Tracestate details to a link.

* SpanContexts are immutable

* Describe IsValid

* formatting

Co-Authored-By: Sergey Kanzhelev <dd2a78e617335ac05535a01af404ce3eb19c30fd@live.com>

* Formmatting

Co-Authored-By: Sergey Kanzhelev <dd2a78e617335ac05535a01af404ce3eb19c30fd@live.com>

* Add SpanContext to TOC

open-telemetry/opentelemetry-specification,open-telemetry/opentelemetry-specification