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/function_definition.cpp

@@ -4,21 +4,16 @@
 
 #include "executable_semantics/ast/function_definition.h"
 
-#include <iostream>
-
 namespace Carbon {
 
-void FunctionDefinition::PrintDepth(int depth) const {
-  std::cout << "fn " << name << " ";
-  PrintExp(param_pattern);
-  std::cout << " -> ";
-  PrintExp(return_type);
+void FunctionDefinition::PrintDepth(int depth, llvm::raw_ostream& out) const {
+  out << "fn " << name << " " << *param_pattern << " -> " << *return_type;
   if (body) {
-    std::cout << " {" << std::endl;
-    PrintStatement(body, depth);
-    std::cout << std::endl << "}" << std::endl;
+    out << " {\n";
+    body->PrintDepth(depth, out);
+    out << "\n}\n";
   } else {
-    std::cout << ";" << std::endl;
+    out << ";\n";
   }
 }