feat(client): `TrySendError<T>` is `From<Error>`

[cratelyn]

this commit introduces a new trait implementation for hyper::client::conn::TrySendError<T>.

this commit allows a TrySendError<T> to be constructed From a conventional hyper::Error.

one example of a motivating use case for this change is that this is needed in order to use interfaces like hyper::client::conn::http2::SendRequest::try_send_request() or hyper::client::conn::http1::SendRequest::try_send_request() in the context of tower middleware; the Service<T> trait's signature is such that the same error type be returned from Service::poll_ready() and Service::call().

thus, the ? operator can construct a TrySendError<T> from errors possibly returned by hyper::client::conn::http2::SendRequest::poll_ready() or hyper::client::conn::http1::SendRequest::poll_ready(), within a Service<T> that eventually calls try_send_request() in the context of Service::call().

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see:

• #3691
• https://docs.rs/hyper/latest/hyper/client/conn/struct.TrySendError.html
• https://docs.rs/hyper/latest/hyper/struct.Error.html
• https://docs.rs/hyper/latest/hyper/client/conn/http2/struct.SendRequest.html#method.try_send_request
• https://docs.rs/tower/latest/tower/trait.Service.html#associatedtype.Error
• https://docs.rs/hyper/latest/hyper/client/conn/http1/struct.SendRequest.html#method.poll_ready
• https://docs.rs/hyper/latest/hyper/client/conn/http2/struct.SendRequest.html#method.poll_ready