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/frame.h

@@ -8,6 +8,7 @@
 #include <list>
 #include <string>
 
+#include "common/ostream.h"
 #include "executable_semantics/interpreter/action.h"
 #include "executable_semantics/interpreter/address.h"
 #include "executable_semantics/interpreter/dictionary.h"
@@ -45,9 +46,9 @@ struct Frame {
 
   Frame(std::string n, Stack<Scope*> s, Stack<Action*> c)
       : name(std::move(std::move(n))), scopes(s), todo(c), continuation() {}
-};
 
-void PrintFrame(Frame* frame, std::ostream& out);
+  void Print(llvm::raw_ostream& out) const;
+};
 
 }  // namespace Carbon