add digit scanning function to scanner

cig.h

@@ -203,6 +203,7 @@ typedef struct error_node {
 } error_node_t;
 
 typedef union scan_value {
+	int digit;
 	uint8_t u8;
 	int8_t i8;
 	uint16_t u16;
@@ -233,6 +234,7 @@ void scanner_error_and_recover(scanner_t *s, const char *message);
 bool scan_eof(scanner_t *s);
 bool scan_literal(scanner_t *s, const char *lit);
 bool scan_whitespace(scanner_t *s);
+bool scan_digit(scanner_t *s);
 bool scan_i64(scanner_t *s);
 bool scan_i32(scanner_t *s);
 bool scan_i16(scanner_t *s);

scanner.c

@@ -82,6 +82,15 @@ bool scan_whitespace(scanner_t *s) {
 	return save != s->cur;
 }
 
+bool scan_digit(scanner_t *s) {
+	if (!isdigit((unsigned char)*s->cur)) {
+		return false;
+	}
+	s->value.digit = (*s->cur) - '0';
+	s->cur++;
+	return true;
+}
+
 bool scan_i64(scanner_t *s) {
 	const char *save = s->cur;
 	if (*s->cur == '-' || *s->cur == '+') s->cur++;