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/address.h

@@ -6,10 +6,10 @@
 #define EXECUTABLE_SEMANTICS_INTERPRETER_ADDRESS_H_
 
 #include <cstdint>
-#include <iostream>
 #include <string>
 #include <vector>
 
+#include "common/ostream.h"
 #include "executable_semantics/interpreter/field_path.h"
 
 namespace Carbon {
@@ -38,9 +38,8 @@ class Address {
   // Currently, that representation consists of an integer index identifying
   // the whole memory allocation, and an optional FieldPath specifying a
   // particular field within that allocation.
-  friend auto operator<<(std::ostream& out, const Address& a) -> std::ostream& {
-    out << "Address(" << a.index << ")" << a.field_path;
-    return out;
+  void Print(llvm::raw_ostream& out) const {
+    out << "Address(" << index << ")" << field_path;
   }
 
   // If *this represents the address of an object with a field named