Express prelude using Carbon syntax (#955)

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

executable_semantics/syntax/lexer.lpp

@@ -83,6 +83,7 @@ WHILE                "while"
 
 /* This should be kept table-like, but isn't automatic due to spaces. */
 identifier            [A-Za-z_][A-Za-z0-9_]*
+intrinsic_identifier  __intrinsic_[A-Za-z0-9_]*
 sized_type_literal    [iuf][1-9][0-9]*
 integer_literal       [0-9]+
 horizontal_whitespace [ \t\r]
@@ -226,7 +227,15 @@ string_literal        \"([^\\\"\n\v\f\r]|\\.)*\"
  /* `*` operator case 5: */
 <INITIAL,AFTER_OPERAND>"*" { return SIMPLE_TOKEN(UNARY_STAR); }
 
-{sized_type_literal} { return ARG_TOKEN(sized_type_literal, yytext); }
+{sized_type_literal} {
+  BEGIN(AFTER_OPERAND);
+  return ARG_TOKEN(sized_type_literal, yytext);
+}
+
+{intrinsic_identifier} {
+  BEGIN(AFTER_OPERAND);
+  return ARG_TOKEN(intrinsic_identifier, yytext);
+}
 
 {identifier} {
   BEGIN(AFTER_OPERAND);

executable_semantics/syntax/parser.ypp

@@ -89,6 +89,7 @@
 
 %token <int> integer_literal
 %token <std::string> identifier
+%token <std::string> intrinsic_identifier
 %token <std::string> sized_type_literal
 %token <std::string> string_literal
 %type <std::string> designator
@@ -302,6 +303,8 @@ expression:
 | paren_expression { $$ = $1; }
 | struct_literal { $$ = $1; }
 | struct_type_literal { $$ = $1; }
+| intrinsic_identifier tuple
+    { $$ = arena->New<IntrinsicExpression>($1, $2, context.source_loc()); }
 | expression EQUAL_EQUAL expression
     {
       $$ = arena->New<PrimitiveOperatorExpression>(