[ox] Cleanup string len handling

Remove UTF-8 parsing. It is a rare enough need that it should have a specialized call when needed.
Better to have a more optimal length fetch for typical case.

deps/ox/src/ox/mc/write.hpp

@@ -207,7 +207,7 @@ constexpr Error MetalClawWriter<Writer>::field(const char*, const BasicString<Sm
 template<Writer_c Writer>
 template<std::size_t L>
 constexpr Error MetalClawWriter<Writer>::field(const char *name, const IString<L> *val) noexcept {
-	return fieldCString(name, val->data(), val->cap());
+	return fieldCString(name, val->data(), val->len());
 }
 
 template<Writer_c Writer>