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/main.cpp

@@ -12,6 +12,10 @@
 #include "llvm/Support/CommandLine.h"
 
 int main(int argc, char* argv[]) {
+  // Printing to stderr should flush stdout. This is most noticeable when stderr
+  // is piped to stdout.
+  llvm::errs().tie(&llvm::outs());
+
   using llvm::cl::desc;
   using llvm::cl::opt;
   opt<bool> trace_option("trace", desc("Enable tracing"));