Add IO#getb_to_end

[HertzDevil]

Resolves #11586. There is also an optimization for IO::Memory that skips IO.copy (a copy is still made so that changes to the IO don't affect the returned Bytes).

[straight-shoota]

I believe the #getb_to_end override needs a call to check_open.

[asterite]

I believe the #getb_to_end override needs a call to check_open.

I'm not sure, gets_to_end doesn't do that. Because it will call read, that will check if it's open.

[straight-shoota]

I mean IO::Memory#getb_to_end.

[asterite]

Looks good!

Maybe offtopic, but will there be other such methods...? For example getb to get a line or up to a delimiter as Bytes.

[HertzDevil]

#getb was indeed briefly mentioned in the original issue, I haven't created a separate issue but a good candidate would be reading bytes delimited by the zero byte.