Parameterized impl declarations (#1189)

* parameterized impls, first step

* bug fixes, comments, etc.

* added another test case, fix a bug in impl lookup

* simplify tests, removing tuple stuff

* don't create impl bindings for -bound implicit parameters

* remove a redundant 'private'

* CamelCase

* add missing backtick

* add a comment

executable_semantics/testdata/impl/fail_ambiguous_impl.carbon

@@ -0,0 +1,52 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{not} %{executable_semantics} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{executable_semantics} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{executable_semantics} %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/impl/fail_ambiguous_impl.carbon:50: ambiguous implementations of interface Vector for class Point
+
+
+package ExecutableSemanticsTest api;
+
+interface Vector {
+  fn Add[me: Self](b: Self) -> Self;
+  fn Scale[me: Self](v: i32) -> Self;
+}
+
+class Point {
+  var x: i32;
+  var y: i32;
+}
+
+external impl Point as Vector {
+  fn Add[me: Point](b: Point) -> Point {
+      return {.x = me.x + b.x, .y = me.y + b.y};
+  }
+  fn Scale[me: Point](v: i32) -> Point {
+      return {.x = me.x * v, .y = me.y * v};
+  }
+}
+
+external impl Point as Vector {
+  fn Add[me: Point](b: Point) -> Point {
+      return {.x = me.x + b.x, .y = me.y + b.y};
+  }
+  fn Scale[me: Point](v: i32) -> Point {
+      return {.x = me.x * v, .y = me.y * v};
+  }
+}
+
+fn AddAndScaleGeneric[T:! Vector](a: T, b: T, s: i32) -> T {
+  return a.Add(b).Scale(s);
+}
+
+fn Main() -> i32 {
+  var a: Point = {.x = 1, .y = 1};
+  var b: Point = {.x = 2, .y = 3};
+  var p: Point = AddAndScaleGeneric(a, b, 5);
+  return p.x - 15;
+}