lang: ast: implement parsing of true/false/null keyword constants

lang/ast/expr/arith.c

@@ -119,6 +119,15 @@ enum ivy_status arith_push_operand(
 		case IVY_KW_CONTINUE:
 			v = ast_node_create(IVY_AST_LOOP_REPEAT);
 			break;
+		case IVY_KW_TRUE:
+			v = ast_node_create(IVY_AST_C_TRUE);
+			break;
+		case IVY_KW_FALSE:
+			v = ast_node_create(IVY_AST_C_FALSE);
+			break;
+		case IVY_KW_NULL:
+			v = ast_node_create(IVY_AST_C_NULL);
+			break;
 		default:
 			return IVY_ERR_BAD_SYNTAX;
 		}

lang/ast/expr/expr.c

@@ -91,6 +91,11 @@ struct ast_node_type expr_node_ops = {
 		KW_PARSER(FINALLY, stmt_parse_end),
 		KW_PARSER(END, stmt_parse_end),
 
+		/* keyword constants */
+		KW_PARSER(TRUE, arith_parse_operand),
+		KW_PARSER(FALSE, arith_parse_operand),
+		KW_PARSER(NULL, arith_parse_operand),
+
 		/* operator/block keywords */
 		KW_PARSER(IN, arith_parse_in),
 		KW_PARSER(IS, arith_parse_operator),

lang/ast/expr/expr.h

@@ -44,6 +44,7 @@ enum expr_component {
 	EXPR_CMP_OPERATOR,
 	EXPR_CMP_OPERAND,
 	EXPR_CMP_MSG,
+	EXPR_CMP_KEYWORD,
 };
 
 struct expr_parser_state {