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/field_path.h

@@ -5,10 +5,11 @@
 #ifndef EXECUTABLE_SEMANTICS_INTERPRETER_FIELD_PATH_H_
 #define EXECUTABLE_SEMANTICS_INTERPRETER_FIELD_PATH_H_
 
-#include <ostream>
 #include <string>
 #include <vector>
 
+#include "common/ostream.h"
+
 namespace Carbon {
 
 // Given some initial Value, a FieldPath identifies a sub-Value within it,
@@ -43,12 +44,10 @@ class FieldPath {
     components.push_back(std::move(name));
   }
 
-  friend auto operator<<(std::ostream& out, const FieldPath& path)
-      -> std::ostream& {
-    for (const std::string& component : path.components) {
+  void Print(llvm::raw_ostream& out) const {
+    for (const std::string& component : components) {
       out << "." << component;
     }
-    return out;
   }
 
  private: