use trailing/leading ones to get length of inlined strings

[tomtomwombat]

Replaces old inline encoding. Trailing bytes of inline strings are 0b11111111, e.g. a len 3 string has 5 trailing u8::MAX. Then, len is calculated using leading/trailing ones:

let addr = self.addr();
    let trailing = if cfg!(target_endian = "little") {
    addr.leading_ones()
} else {
    addr.trailing_ones()
};
WIDTH - (trailing as usize >> 3)

This is simpler than previously, because the start position of utf-8 bytes are always 0 and the new encoding naturally works with 8 inlined bytes. Also, WORD_NUL_MAP is adjusted to (usize::MAX >> ALIGN_BITS * 2) << ALIGN_BITS;:

• it's an impossible utf-8 representation, so doesn't collide with other inlined strings
• it's symmetrical, so it's the same on big or little endian.
• the first 2 leading bits are 0, classifying it as inlined (from TAG_MASK)
• the trailing bits are 0, so leading/trailing 1s is 0, meaning it works with inline_len (although never used)

Inspired from @bonzini and https://github.com/bonzini/arcstring/blob/main/src/encoder.rs