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/parse_and_lex_context.cpp

@@ -4,12 +4,16 @@
 
 #include "executable_semantics/syntax/parse_and_lex_context.h"
 
+namespace Carbon {
+
 // Writes a syntax error diagnostic, containing message, for the input file at
 // the given line, to standard error.
-auto Carbon::ParseAndLexContext::PrintDiagnostic(const std::string& message,
-                                                 int line_num) -> void {
+auto ParseAndLexContext::PrintDiagnostic(const std::string& message,
+                                         int line_num) -> void {
   // TODO: Do we really want this to be fatal?  It makes the comment and the
   // name a lie, and renders some of the other yyparse() result propagation code
   // moot.
   FATAL_COMPILATION_ERROR(line_num) << message;
 }
+
+}  // namespace Carbon