feat(error): revamp hyper::Error type

**The `Error` is now an opaque struct**, which allows for more variants to
be added freely, and the internal representation to change without being
breaking changes.

For inspecting an `Error`, there are several `is_*` methods to check for
certain classes of errors, such as `Error::is_parse()`. The `cause` can
also be inspected, like before. This likely seems like a downgrade, but
more inspection can be added as needed!

The `Error` now knows about more states, which gives much more context
around when a certain error occurs. This is also expressed in the
description and `fmt` messages.

**Most places where a user would provide an error to hyper can now pass
any error type** (`E: Into<Box<std::error::Error>>`). This error is passed
back in relevant places, and can be useful for logging. This should make
it much clearer about what error a user should provide to hyper: any it
feels is relevant!

Closes #1128
Closes #1130
Closes #1431
Closes #1338

BREAKING CHANGE: `Error` is no longer an enum to pattern match over, or
  to construct. Code will need to be updated accordingly.

  For body streams or `Service`s, inference might be unable to determine
  what error type you mean to return. Starting in Rust 1.26, you could
  just label that as `!` if you never return an error.

benches/server.rs

@@ -78,7 +78,7 @@ fn throughput_fixedsize_large_payload(b: &mut test::Bencher) {
 fn throughput_fixedsize_many_chunks(b: &mut test::Bencher) {
     bench_server!(b, ("content-length", "1000000"), || {
         static S: &'static [&'static [u8]] = &[&[b'x'; 1_000] as &[u8]; 1_000] as _;
-        Body::wrap_stream(stream::iter_ok(S.iter()).map(|&s| s))
+        Body::wrap_stream(stream::iter_ok::<_, String>(S.iter()).map(|&s| s))
     })
 }
 
@@ -96,7 +96,7 @@ fn throughput_chunked_large_payload(b: &mut test::Bencher) {
 fn throughput_chunked_many_chunks(b: &mut test::Bencher) {
     bench_server!(b, ("transfer-encoding", "chunked"), || {
         static S: &'static [&'static [u8]] = &[&[b'x'; 1_000] as &[u8]; 1_000] as _;
-        Body::wrap_stream(stream::iter_ok(S.iter()).map(|&s| s))
+        Body::wrap_stream(stream::iter_ok::<_, String>(S.iter()).map(|&s| s))
     })
 }