Generic classes (#1124)

* start of generic classes

* fix regressions

* class functions in interfaces

* access to class function on interface from class parameter

* more stuff working for generic classes

* fixing bugs

* update TypeEqual for generic classes

* fixing bugs and finding new ones

* disable unqualified access to members from other members for now

* minor edits

* bug fixes

* introduce compile_time_value to use in type checker instead of constant_value

* cleanup

* put a CHECK back in

* failure test cases for the new FATAL_COMPILATION_ERROR

* change a runtime FATAL into a FATAL_COMPILATION_ERROR

* Update executable_semantics/ast/declaration.h

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/action_stack.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/interpreter.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/resolve_names.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* responses to reviews

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* rename compile_time_value to symbolic_identity

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/type_checker.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* Update executable_semantics/interpreter/value.cpp

Co-authored-by: Jon Meow <jperkins@google.com>

* more edits from review

* Apply suggestions from code review

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

* review responses

* Update executable_semantics/interpreter/interpreter.cpp

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

* const impl_scope for TypeCheckChoiceDeclaration

* add some const

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

executable_semantics/interpreter/impl_scope.cpp

@@ -6,6 +6,7 @@
 
 #include "executable_semantics/common/error.h"
 #include "executable_semantics/interpreter/value.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Casting.h"
 
 using llvm::cast;
@@ -79,4 +80,17 @@ auto ImplScope::ResolveHere(Nonnull<const Value*> iface_type,
   }
 }
 
+// TODO: Add indentation when printing the parents.
+void ImplScope::Print(llvm::raw_ostream& out) const {
+  out << "impls: ";
+  llvm::ListSeparator sep;
+  for (Impl impl : impls_) {
+    out << sep << *(impl.type) << " as " << *(impl.interface);
+  }
+  out << "\n";
+  for (const Nonnull<const ImplScope*>& parent : parent_scopes_) {
+    out << *parent;
+  }
+}
+
 }  // namespace Carbon