Optimize String#rchop?()

[HertzDevil]

This is similar to #15153.

• Uses Char::Reader to read backwards from the end of the string. This does not require decoding the whole string, unlike calculating size - 1.
• If the current string does not end with an ASCII character and its size is unknown, #single_byte_optimizable? traverses the entire string. This is no longer unnecessary because the above handles everything already.
• If the current string's size is known, the new string's size can be derived from it.

[straight-shoota]

I love that this is an optimization which even reduces code complexity ❤️