dap: implement variable references

Implement variable references to inspect the state of a stack frame.

Variable reference ids are composed of two sections. A thread mask that
is the first 8 bytes and the remainder is an increasing number that gets
reset each time a thread is resumed. This allows the adapter to know
which thread to delegate the variables request to and allows the
variable references to still remain confined to each thread. An int32 is
used for this because variable references need to be in the range of
(0, 2^32).

At the moment, only the platform variables and some of the exec
operations for an operation. These are labeled as "arguments" to the
stack frame.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

dap/server.go

@@ -125,6 +125,10 @@ func (s *Server) handleMessage(c Context, m dap.Message) (dap.ResponseMessage, e
 		return s.h.Threads.Do(c, req)
 	case *dap.StackTraceRequest:
 		return s.h.StackTrace.Do(c, req)
+	case *dap.ScopesRequest:
+		return s.h.Scopes.Do(c, req)
+	case *dap.VariablesRequest:
+		return s.h.Variables.Do(c, req)
 	case *dap.EvaluateRequest:
 		return s.h.Evaluate.Do(c, req)
 	case *dap.SourceRequest: