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/interpreter/action.h

@@ -5,9 +5,9 @@
 #ifndef EXECUTABLE_SEMANTICS_INTERPRETER_ACTION_H_
 #define EXECUTABLE_SEMANTICS_INTERPRETER_ACTION_H_
 
-#include <iostream>
 #include <vector>
 
+#include "common/ostream.h"
 #include "executable_semantics/ast/expression.h"
 #include "executable_semantics/ast/statement.h"
 #include "executable_semantics/interpreter/stack.h"
@@ -48,14 +48,14 @@ struct Action {
   static auto MakeStatementAction(const Statement* s) -> Action*;
   static auto MakeValAction(const Value* v) -> Action*;
 
-  static void PrintList(Stack<Action*> ls, std::ostream& out);
+  static void PrintList(Stack<Action*> ls, llvm::raw_ostream& out);
 
   auto GetLValAction() const -> const LValAction&;
   auto GetExpressionAction() const -> const ExpressionAction&;
   auto GetStatementAction() const -> const StatementAction&;
   auto GetValAction() const -> const ValAction&;
 
-  void Print(std::ostream& out);
+  void Print(llvm::raw_ostream& out) const;
 
   inline auto tag() const -> ActionKind {
     return std::visit([](const auto& t) { return t.Kind; }, value);