fix: Make arguments of SpanBase's '==' of type Self (open-telemetry#614)

Sources/OpenTelemetryApi/Trace/Span.swift

@@ -113,7 +113,7 @@ public extension SpanBase {
         hasher.combine(context.spanId)
     }
 
-    static func == (lhs: SpanBase, rhs: SpanBase) -> Bool {
+    static func == (lhs: Self, rhs: Self) -> Bool {
         return lhs.context.spanId == rhs.context.spanId
     }
 }