Refactor output to be more streaming-focused. (#666)

- Switch code to llvm::raw_ostream as part of standardizing output forms.
    - Preferring llvm::raw_ostream over std::ostream because other tooling code should be expected to rely on llvm more closely, and an overall preference towards library consistency.
    - There are a couple spots in syntax/ that still use std streams, but I'd prefer to take a separate PR to see how best to address those.
    - std::boolalpha doesn't work with llvm, so I've implemented equivalent in a couple places (not enough that it felt like worth making a helper function).
- Implement Print(ostream) as consistently as we can, as an instance member.
    - This facilitates the use of the common/ostream.h template to provide operators.
    - Preferring this approach so that Print is easily accessible via gdb, per suggestion on #executable-semantics.
- Switch code currently calling `type->Print(ostream)` to instead do `ostream << *type`.
- Remove the unused `PrintTypeEnv`, nothing used it and the declaration didn't match the definition.

executable_semantics/ast/member.cpp

@@ -4,8 +4,6 @@
 
 #include "executable_semantics/ast/member.h"
 
-#include <iostream>
-
 namespace Carbon {
 
 auto Member::MakeFieldMember(int line_num, std::string name,
@@ -20,13 +18,11 @@ auto Member::GetFieldMember() const -> const FieldMember& {
   return std::get<FieldMember>(value);
 }
 
-void Member::Print() {
+void Member::Print(llvm::raw_ostream& out) const {
   switch (tag()) {
     case MemberKind::FieldMember:
       const auto& field = GetFieldMember();
-      std::cout << "var " << field.name << " : ";
-      PrintExp(field.type);
-      std::cout << ";" << std::endl;
+      out << "var " << field.name << " : " << *field.type << ";\n";
       break;
   }
 }