Stop allocating sub-Values on the Heap. (#648)

This minimizes use of the Heap, and moves us toward not using it at compile time.

executable_semantics/ast/BUILD

@@ -17,6 +17,7 @@ cc_library(
         ":function_definition",
         ":member",
         ":struct_definition",
+        "//executable_semantics/interpreter:address",
         "//executable_semantics/interpreter:containers",
     ],
 )

executable_semantics/ast/declaration.h

@@ -11,6 +11,7 @@
 #include "executable_semantics/ast/function_definition.h"
 #include "executable_semantics/ast/member.h"
 #include "executable_semantics/ast/struct_definition.h"
+#include "executable_semantics/interpreter/address.h"
 #include "executable_semantics/interpreter/dictionary.h"
 
 namespace yy {
@@ -21,7 +22,6 @@ namespace Carbon {
 
 struct Value;
 
-using Address = unsigned int;
 using TypeEnv = Dictionary<std::string, const Value*>;
 using Env = Dictionary<std::string, Address>;