Rewrite std::timer::recv_timeout more clearly

[catamorphism]

As per a FIXME. (Though I have to say, the use of the either::either combinator seems perfectly cromulent to me...)

[bblum]

I'd be more worried by the unsafe transmute-to-mut above, which originates from a bunch of places @Port can be generated in net_tcp.

But I understand timer and net_tcp are both deprecated pending #6435.