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/statement.h

@@ -7,6 +7,7 @@
 
 #include <list>
 
+#include "common/ostream.h"
 #include "executable_semantics/ast/expression.h"
 
 namespace Carbon {
@@ -162,6 +163,8 @@ struct Statement {
   auto GetRun() const -> const Run&;
   auto GetAwait() const -> const Await&;
 
+  void PrintDepth(int depth, llvm::raw_ostream& out) const;
+
   inline auto tag() const -> StatementKind {
     return std::visit([](const auto& t) { return t.Kind; }, value);
   }
@@ -175,8 +178,6 @@ struct Statement {
       value;
 };
 
-void PrintStatement(const Statement*, int);
-
 }  // namespace Carbon
 
 #endif  // EXECUTABLE_SEMANTICS_AST_STATEMENT_H_