Add name accessor to NamedEntityView (#994)

This required changing BindingPattern::name() to return `"_"` instead of nullopt when representing a `"_"` binding.

Semi-related drive-by fixes:
- Update BindingPlaceholderValue to expose a NamedEntity instead of a string name, and stop exposing its type.
- Drop the unused SourceLocation parameter of ValueEqual

executable_semantics/ast/statement.cpp

@@ -100,7 +100,7 @@ void Statement::PrintDepth(int depth, llvm::raw_ostream& out) const {
     }
     case StatementKind::Continuation: {
       const auto& cont = cast<Continuation>(*this);
-      out << "continuation " << cont.continuation_variable() << " ";
+      out << "continuation " << cont.name() << " ";
       if (depth < 0 || depth > 1) {
         out << "\n";
       }