Add 56-bit SWAR (SIMD Within A Register) acceleration to skip_length/2 and byte_size_remaining_at/2, processing 8 ASCII bytes per iteration instead of one. Uses the Mycroft zero-byte detection algorithm to validate that 7+1 bytes are all ASCII with no \r in a single guard. This mirrors the approach taken in OTP's string module (OTP https://github.com/erlang/otp/pull/10948).