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.cpp

@@ -4,6 +4,7 @@
 
 #include "executable_semantics/syntax/syntax_helpers.h"
 
+#include "common/check.h"
 #include "common/ostream.h"
 #include "executable_semantics/common/arena.h"
 #include "executable_semantics/common/tracing_flag.h"
@@ -12,7 +13,29 @@
 
 namespace Carbon {
 
-void ExecProgram(const std::list<const Declaration*>& fs) {
+// Adds builtins, currently only Print(). Note Print() is experimental, not
+// standardized, but is made available for printing state in tests.
+static void AddIntrinsics(std::list<const Declaration*>* fs) {
+  std::vector<TuplePattern::Field> print_fields = {TuplePattern::Field(
+      "0", global_arena->New<BindingPattern>(
+               -1, "format_str",
+               global_arena->New<ExpressionPattern>(
+                   global_arena->New<StringTypeLiteral>(-1))))};
+  auto* print_return =
+      global_arena->New<Return>(-1,
+                                global_arena->New<IntrinsicExpression>(
+                                    IntrinsicExpression::IntrinsicKind::Print),
+                                false);
+  auto* print = global_arena->New<FunctionDeclaration>(FunctionDefinition(
+      -1, "Print", std::vector<GenericBinding>(),
+      global_arena->New<TuplePattern>(-1, print_fields),
+      global_arena->New<ExpressionPattern>(global_arena->New<TupleLiteral>(-1)),
+      /*is_omitted_return_type=*/false, print_return));
+  fs->insert(fs->begin(), print);
+}
+
+void ExecProgram(std::list<const Declaration*> fs) {
+  AddIntrinsics(&fs);
   if (tracing_output) {
     llvm::outs() << "********** source program **********\n";
     for (const auto* decl : fs) {