feat(client): add a new Client struct with super powers

- Includes ergonomic traits like IntoUrl and IntoBody, allowing easy
usage.
- Client can have a RedirectPolicy.
- Client can have a SslVerifier.

Updated benchmarks for client. (Disabled rust-http client bench since it
hangs.)

Closes #62 
I think this is a good specialization to avoid dynamic dispatch, but it may be useful to set constant-size readers from non-BufReaders, like Files.