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/declaration.h

@@ -8,6 +8,7 @@
 #include <list>
 #include <string>
 
+#include "common/ostream.h"
 #include "executable_semantics/ast/function_definition.h"
 #include "executable_semantics/ast/member.h"
 #include "executable_semantics/ast/struct_definition.h"
@@ -82,7 +83,7 @@ class Declaration {
   auto GetChoiceDeclaration() const -> const ChoiceDeclaration&;
   auto GetVariableDeclaration() const -> const VariableDeclaration&;
 
-  void Print() const;
+  void Print(llvm::raw_ostream& out) const;
 
   inline auto tag() const -> DeclarationKind {
     return std::visit([](const auto& t) { return t.Kind; }, value);