Express prelude using Carbon syntax (#955)

This should be clearer, more maintainable, and more scalable than building the prelude AST by hand.

executable_semantics/interpreter/exec_program.cpp

@@ -4,6 +4,8 @@
 
 #include "executable_semantics/interpreter/exec_program.h"
 
+#include <variant>
+
 #include "common/check.h"
 #include "common/ostream.h"
 #include "executable_semantics/common/arena.h"
@@ -14,30 +16,7 @@
 
 namespace Carbon {
 
-// Adds builtins, currently only Print(). Note Print() is experimental, not
-// standardized, but is made available for printing state in tests.
-static void AddIntrinsics(Nonnull<Arena*> arena,
-                          std::vector<Nonnull<Declaration*>>* declarations) {
-  SourceLocation source_loc("<intrinsic>", 0);
-  std::vector<Nonnull<Pattern*>> print_params = {arena->New<BindingPattern>(
-      source_loc, "format_str",
-      arena->New<ExpressionPattern>(
-          arena->New<StringTypeLiteral>(source_loc)))};
-  auto print_return = arena->New<Block>(
-      source_loc, std::vector<Nonnull<Statement*>>({arena->New<Return>(
-                      source_loc,
-                      arena->New<IntrinsicExpression>(
-                          IntrinsicExpression::Intrinsic::Print),
-                      false)}));
-  auto print = arena->New<FunctionDeclaration>(
-      source_loc, "Print", std::vector<Nonnull<GenericBinding*>>(),
-      arena->New<TuplePattern>(source_loc, print_params),
-      ReturnTerm::Explicit(arena->New<TupleLiteral>(source_loc)), print_return);
-  declarations->insert(declarations->begin(), print);
-}
-
 void ExecProgram(Nonnull<Arena*> arena, AST ast, bool trace) {
-  AddIntrinsics(arena, &ast.declarations);
   if (trace) {
     llvm::outs() << "********** source program **********\n";
     for (const auto decl : ast.declarations) {