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/syntax/syntax_helpers.h

@@ -16,7 +16,7 @@
 namespace Carbon {
 
 // Runs the top-level declaration list.
-void ExecProgram(const std::list<const Declaration*>& fs);
+void ExecProgram(std::list<const Declaration*> fs);
 
 }  // namespace Carbon