Merge HandleValue into the Step functions (#598)

* moved most of logic from HandleValue into the Step functions

* removed the HandleValue function

* added some comments

* responses to Geoffrey

* move increment of act->pos out of the step logic for value actions

* changed act->pos to start at 0 and not have any gaps

* Update executable_semantics/interpreter/action.h

Co-authored-by: Geoff Romer <gromer@google.com>

* update comment based on Geoffrey's suggestion

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/interpreter/action.cpp

@@ -63,7 +63,7 @@ auto MakeExpAct(const Expression* e) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::ExpressionAction;
   act->u.exp = e;
-  act->pos = -1;
+  act->pos = 0;
   return act;
 }
 
@@ -71,7 +71,7 @@ auto MakeLvalAct(const Expression* e) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::LValAction;
   act->u.exp = e;
-  act->pos = -1;
+  act->pos = 0;
   return act;
 }
 
@@ -79,7 +79,7 @@ auto MakeStmtAct(const Statement* s) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::StatementAction;
   act->u.stmt = s;
-  act->pos = -1;
+  act->pos = 0;
   return act;
 }
 
@@ -87,21 +87,21 @@ auto MakeValAct(const Value* v) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::ValAction;
   act->u.val = v;
-  act->pos = -1;
+  act->pos = 0;
   return act;
 }
 
 auto MakeExpToLvalAct() -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::ExpToLValAction;
-  act->pos = -1;
+  act->pos = 0;
   return act;
 }
 
 auto MakeDeleteAct(Address a) -> Action* {
   auto* act = new Action();
   act->tag = ActionKind::DeleteTmpAction;
-  act->pos = -1;
+  act->pos = 0;
   act->u.delete_tmp = a;
   return act;
 }