Relax lifetime bounds on Reader/Writer impls for trait boxes

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>`
for 'a other than 'static.
auto merge of #16843 : bkoropoff/rust/reader-writer-box, r=alexcrichton

Cargo needs this to be able to instantiate `TerminfoTerminal<Box<Writer+'a>>` for 'a other than 'static.