interfaces, impls, and constrained generics (basics) (#1073)

* interfaces, impls, and constrained generics (basics)

* separate type checking into declare vs. type check, removing redundancy

* external impls

* added impl scopes to handle generics calling generics

* cleanup

* more cleanup

* Update executable_semantics/testdata/interface/external_impl_point_vector.carbon

Co-authored-by: josh11b <josh11b@users.noreply.github.com>

* Update executable_semantics/testdata/interface/generic_call_generic.carbon

Co-authored-by: josh11b <josh11b@users.noreply.github.com>

* Update executable_semantics/testdata/interface/tuple_vector_add_scale.carbon

Co-authored-by: josh11b <josh11b@users.noreply.github.com>

* Update executable_semantics/testdata/interface/vector_point_add_scale.carbon

Co-authored-by: josh11b <josh11b@users.noreply.github.com>

* change ImplementationDeclaration to ImplDeclaration

* remove impl_type_value

* split NamedEntity into two

* changed GetName to be a free function

* adding comments

* more edits to respond to review

* introduce ImplBinding, remove punning on GenericBinding

* new test case and some minor edits

* refactor GetMember and GetField to move impl logic to interpreter

* remove commennt

* change EntityView to ImplBinding in FieldAccess...

* move ImplBinding

* review response

* added example to impl_scope.h

* minor edits

* Update executable_semantics/interpreter/field_path.h

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

* Update executable_semantics/interpreter/value.cpp

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

* Update executable_semantics/interpreter/interpreter.cpp

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

* Update executable_semantics/ast/expression.h

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

* Update executable_semantics/ast/expression.h

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

* Update executable_semantics/ast/generic_binding.h

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

* more edits from review

* review response

* Update executable_semantics/ast/static_scope.h

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

* remove ImplType, renamed node_view to value_node

Co-authored-by: josh11b <josh11b@users.noreply.github.com>
Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/interpreter/BUILD

@@ -85,6 +85,8 @@ cc_library(
     hdrs = ["field_path.h"],
     deps = [
         "//common:ostream",
+        "//executable_semantics/ast",
+        "//executable_semantics/ast:static_scope",
         "@llvm-project//llvm:Support",
     ],
 )
@@ -183,8 +185,14 @@ cc_library(
 
 cc_library(
     name = "type_checker",
-    srcs = ["type_checker.cpp"],
-    hdrs = ["type_checker.h"],
+    srcs = [
+        "impl_scope.cpp",
+        "type_checker.cpp",
+    ],
+    hdrs = [
+        "impl_scope.h",
+        "type_checker.h",
+    ],
     deps = [
         ":action_and_value",
         ":dictionary",