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.h

@@ -5,6 +5,7 @@
 #ifndef EXECUTABLE_SEMANTICS_AST_FUNCTION_DEFINITION_H_
 #define EXECUTABLE_SEMANTICS_AST_FUNCTION_DEFINITION_H_
 
+#include "common/ostream.h"
 #include "executable_semantics/ast/expression.h"
 #include "executable_semantics/ast/statement.h"
 
@@ -21,8 +22,8 @@ struct FunctionDefinition {
         return_type(return_type),
         body(body) {}
 
-  void Print() const { PrintDepth(-1); }
-  void PrintDepth(int depth) const;
+  void Print(llvm::raw_ostream& out) const { PrintDepth(-1, out); }
+  void PrintDepth(int depth, llvm::raw_ostream& out) const;
 
   int line_num;
   std::string name;