Ben Morrow noticed that:
PerlIO_push is exported, but PerlIO_find_layer is not, so getting a
PerlIO_funcs* to pass to it is difficult.
Several exported functions (PerlIO_parse_layers, PerlIO_layer_fetch,
PerlIO_list_free, ...) take a PerlIO_list_t* argument, but
PerlIO_list_alloc is not exported, so these are effectively useless.
PerlIO_arg_fetch
PerlIO_debug
PerlIO_define_layer
+ PerlIO_find_layer
PerlIO_isutf8
PerlIO_layer_fetch
+ PerlIO_list_alloc
PerlIO_list_free
PerlIO_modestr
PerlIO_parse_layers