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/fuzzing/ast_to_proto.cpp

@@ -80,6 +80,10 @@ static auto ExpressionToProto(const Expression& expression)
     -> Fuzzing::Expression {
   Fuzzing::Expression expression_proto;
   switch (expression.kind()) {
+    case ExpressionKind::InstantiateImpl: {
+      // UNDER CONSTRUCTION
+      break;
+    }
     case ExpressionKind::CallExpression: {
       const auto& call = cast<CallExpression>(expression);
       auto* call_proto = expression_proto.mutable_call();