std: Allow ?Sized parameters in std::io::copy
std: Allow ?Sized parameters in std::io::copy

std::io::copy did not allow passing trait objects directly (only with an
extra &mut wrapping).

graydon/rust,pshc/rust,pshc/rust,GBGamer/rust,dwillmer/rust,pshc/rust,GBGamer/rust,graydon/rust,pshc/rust,dwillmer/rust,pshc/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,pshc/rust,dwillmer/rust,dwillmer/rust,graydon/rust,GBGamer/rust,aidancully/rust,pshc/rust,aidancully/rust,GBGamer/rust,pshc/rust,dwillmer/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,dwillmer/rust,dwillmer/rust