Alias .string to .to_s

[sebyx07]

imo, a lot of users would rather call buffer.to_s instead of the .string method, nobody atm depends on the .to_s. There already is .inspect that behaves like the current to_s

https://bugs.ruby-lang.org/issues/20703?next_issue_id=20702

I can do a pr, but lmk if it's a good idea

[kou]

I'm not sure whether the change is safe or not but let's try.

[sebyx07]

also added .to_str https://bugs.ruby-lang.org/issues/20703