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/heap.h

@@ -5,9 +5,9 @@
 #ifndef EXECUTABLE_SEMANTICS_INTERPRETER_MEMORY_H_
 #define EXECUTABLE_SEMANTICS_INTERPRETER_MEMORY_H_
 
-#include <ostream>
 #include <vector>
 
+#include "common/ostream.h"
 #include "executable_semantics/interpreter/address.h"
 #include "executable_semantics/interpreter/value.h"
 
@@ -28,19 +28,19 @@ class Heap {
 
   // Writes the given value at the address in the heap after
   // checking that the address is alive.
-  auto Write(const Address& a, const Value* v, int line_num) -> void;
+  void Write(const Address& a, const Value* v, int line_num);
 
   // Put the given value on the heap and mark it as alive.
   auto AllocateValue(const Value* v) -> Address;
 
   // Marks the object at this address, and all of its sub-objects, as dead.
-  auto Deallocate(const Address& address) -> void;
+  void Deallocate(const Address& address);
 
   // Print the value at the given address to the stream `out`.
-  auto PrintAddress(const Address& a, std::ostream& out) -> void;
+  void PrintAddress(const Address& a, llvm::raw_ostream& out) const;
 
   // Print all the values on the heap to the stream `out`.
-  auto PrintHeap(std::ostream& out) -> void;
+  void Print(llvm::raw_ostream& out) const;
 
  private:
   // Signal an error if the address is no longer alive.