simdutf: update to 8.0.0

This patch updates vendored simdutf to 8.0.0:
https://github.com/simdutf/simdutf/releases/tag/v8.0.0, and makes use
of simdutf's own C API.

Author: TerrorJack

cbits/validate_utf8.cpp

@@ -0,0 +1,5 @@
+#include "simdutf_c.h"
+
+bool _hs_text_is_valid_utf8_offset(const char *buf, size_t off, size_t len) {
+  return simdutf_validate_utf8(buf + off, len);
+}