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/value.h

@@ -11,6 +11,7 @@
 #include <variant>
 #include <vector>
 
+#include "common/ostream.h"
 #include "executable_semantics/ast/statement.h"
 #include "executable_semantics/interpreter/address.h"
 #include "executable_semantics/interpreter/field_path.h"
@@ -229,6 +230,8 @@ struct Value {
   auto SetField(const FieldPath& path, const Value* field_value,
                 int line_num) const -> const Value*;
 
+  void Print(llvm::raw_ostream& out) const;
+
  private:
   std::variant<IntValue, FunctionValue, PointerValue, BoolValue, StructValue,
                AlternativeValue, TupleValue, IntType, BoolType, TypeType,
@@ -238,8 +241,6 @@ struct Value {
       value;
 };
 
-void PrintValue(const Value* val, std::ostream& out);
-
 auto CopyVal(const Value* val, int line_num) -> const Value*;
 
 auto TypeEqual(const Value* t1, const Value* t2) -> bool;