Function _to_utf8_lower_flags 

pub unsafe fn _to_utf8_lower_flags(
    my_perl: *mut PerlInterpreter,
    a: *const U8,
    b: *const U8,
    c: *mut U8,
    d: *mut STRLEN,
    e: bool,
) -> UV

thx shim for crate::_to_utf8_lower_flags.