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.cpp

@@ -10,10 +10,9 @@
 
 namespace Carbon {
 
-void PrintFrame(Frame* frame, std::ostream& out) {
-  out << frame->name;
-  out << "{";
-  Action::PrintList(frame->todo, out);
+void Frame::Print(llvm::raw_ostream& out) const {
+  out << name << "{";
+  Action::PrintList(todo, out);
   out << "}";
 }