Explanation Conclusion should include non-retrievable concepts (#157)

## What is the goal of this PR?

Introduce a different answer type for conclusions inside explanations. This permits returning any concept, not only retrievable concepts, which is important for returning a thorough explanation due to unification.

## What are the changes implemented in this PR?

- Swap `ConceptMap` for a `map`

Author: jmsfltchr

common/BUILD

@@ -33,7 +33,10 @@ proto_library(
 proto_library(
     name = "logic-proto",
     srcs = ["logic.proto"],
-    deps = [":answer-proto"]
+    deps = [
+        ":answer-proto",
+        ":concept-proto"
+    ],
 )
 
 proto_library(

common/logic.proto

@@ -21,6 +21,7 @@ option java_package = "com.vaticle.typedb.protocol";
 option java_outer_classname = "LogicProto";
 
 import "common/answer.proto";
+import "common/concept.proto";
 
 
 package typedb.protocol;
@@ -114,7 +115,7 @@ message Explanation {
     Rule rule = 1;
     map<string, VarList> var_mapping = 2;
     ConceptMap condition = 3;
-    ConceptMap conclusion = 4;
+    map<string, Concept> conclusion = 4;
 
     message VarList {
         repeated string vars = 1;