Add a new kind of Witness value that carries an expression (#1324)

This renames `Witness` to `ImplWitness` and adds a new form, `SymbolicWitness`, that holds an expression by which a witness can be computed. A common base class `Witness` is provided.

We form the new kind of witness when evaluation of a witness expression fails because the witness is not in scope, as happens when evaluating a subexpression such as a type expression in isolation, and retry evaluation in the larger context when the interpreter performs type instantiation when running the code.

This allows us to properly handle compile-time evaluation of constructs involving witness table lookups when the witness can be statically determined. The intent is that we will eventually also form symbolic witness table references when impl selection finds a non-final witness, in order to support specialization.

Simplify `NominalClassValue`: it can now always store a witness map rather than either a witness map or a witness-or-witness-expression map.

explorer/interpreter/action_stack.cpp

@@ -124,6 +124,7 @@ auto ActionStack::FinishAction() -> ErrorOr<Success> {
       CARBON_FATAL() << "ScopeAction at top of stack";
     case Action::Kind::StatementAction:
     case Action::Kind::DeclarationAction:
+    case Action::Kind::RecursiveAction:
       PopScopes();
   }
   return Success();
@@ -135,6 +136,7 @@ auto ActionStack::FinishAction(Nonnull<const Value*> result)
   switch (act->kind()) {
     case Action::Kind::StatementAction:
     case Action::Kind::DeclarationAction:
+    case Action::Kind::RecursiveAction:
       CARBON_FATAL() << "This kind of Action cannot produce results: " << *act;
     case Action::Kind::ScopeAction:
       CARBON_FATAL() << "ScopeAction at top of stack";