Add string parsing and a print builtin (#721)

It was in my mind to add String in order to support libraries in `package`.  `print` is added in order to have a String go to stdout. I've tried to do `print` in a way that won't be too hard to add other printable types, but it's probably also somewhat optional here -- that is, if desired, I could remove it. But it was a lot easier to doublecheck `\n` behavior with it, and I suspect it'll be helpful in other tests if it supports more value types.

On the side, this also fixes dereferencing in Pattern/Expression Print() calls, which I was noticing printing pointers instead of values. This may be another argument for moving away from passing pointers, since this seems to be a difficult-to-catch error.

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/pattern.cpp

@@ -37,19 +37,19 @@ void Pattern::Print(llvm::raw_ostream& out) const {
       out << "(";
       llvm::ListSeparator sep;
       for (const TuplePattern::Field& field : tuple.Fields()) {
-        out << sep << field.name << " = " << field.pattern;
+        out << sep << field.name << " = " << *field.pattern;
       }
       out << ")";
       break;
     }
     case Kind::AlternativePattern: {
       const auto& alternative = cast<AlternativePattern>(*this);
-      out << alternative.ChoiceType() << "." << alternative.AlternativeName()
-          << alternative.Arguments();
+      out << *alternative.ChoiceType() << "." << alternative.AlternativeName()
+          << *alternative.Arguments();
       break;
     }
     case Kind::ExpressionPattern:
-      out << cast<ExpressionPattern>(*this).Expression();
+      out << *cast<ExpressionPattern>(*this).Expression();
       break;
   }
 }