The maintenance burden is not worth the advantages it provides in
limited situations.
Moreover, if other decoders are to be implemented, most multi-byte
schemes would not be able to support a functional interface of similar
simplicity, and single-byte schemes wouldn't benefit much
The Intl tests avoided one mission user-function calls by doing a simple
loop, something unlikely to be used in real-world situations; wrapping
the test in a generator adds the overhead one would expect to have,
making the pure PHP implementation much more competitive