Fix OSS-Fuzz #508087118: avoid stack overflow in str2tag

str2tag_core is recursive (one frame per character), so a long runtime
input such as a fuzzer-supplied Content-Type would overflow the stack.
Rewrite the runtime entry point str2tag() iteratively while keeping the
recursive constexpr str2tag_core for compile-time UDL evaluation. The
hash output is unchanged for all inputs.