File#read_at limited to Int32

[Blacksmoke16]

Currently https://github.com/crystal-lang/crystal/blob/master/src/file/preader.cr#L7's ivars are typed as Int32, this prevents reading a large enough file at points greater than Int32::Max.

Similarly, thoughts on making bytesize be nilable? This would support the use case of "read the remainder of a file starting at offset x" when using https://crystal-lang.org/api/master/File.html#read_at(offset,bytesize,&)-instance-method.

[asterite]

Yes for the first thing, I don't know about the second one, maybe it's okay.