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/parser.ypp

@@ -90,6 +90,7 @@ void Carbon::Parser::error(const location_type&, const std::string& message) {
 %token <int> integer_literal
 %token <std::string> identifier
 %token <std::string> sized_type_literal
+%token <std::string> string_literal
 %type <std::string> designator
 %type <const Declaration*> declaration
 %type <FunctionDefinition> function_declaration
@@ -131,6 +132,7 @@ void Carbon::Parser::error(const location_type&, const std::string& message) {
 %token AND
 %token OR
 %token NOT
+%token STRING
 %token BOOL
 %token TYPE
 %token FN
@@ -218,6 +220,8 @@ expression:
     { $$ = global_arena->New<IndexExpression>(yylineno, $1, $3); }
 | integer_literal
     { $$ = global_arena->New<IntLiteral>(yylineno, $1); }
+| string_literal
+    { $$ = global_arena->New<StringLiteral>(yylineno, $1); }
 | TRUE
     { $$ = global_arena->New<BoolLiteral>(yylineno, true); }
 | FALSE
@@ -229,6 +233,8 @@ expression:
       CHECK($1[0] == 'i' && val == 32)  << "Only i32 is supported for now: " << $1;
       $$ = global_arena->New<IntTypeLiteral>(yylineno);
     }
+| STRING
+    { $$ = global_arena->New<StringTypeLiteral>(yylineno); }
 | BOOL
     { $$ = global_arena->New<BoolTypeLiteral>(yylineno); }
 | TYPE