.. turns out that, without the export, the modules aren't accessible
outside of the crate, itself. I thought that, by importing some module
into another (nesting it) and exporting from that nested module (which
is, itself, exported from std.rc) that my mod would be in the build
artifact. This doesn't appear to be the case. learning is fun!

.. going to rework the listen() API to be non-blocking.

also whitespace cleanup

.. for now, the test just spins up the server and listens for messages,
echoing them back to an output port. there's a "kill" msg that it will
listen for. need to point the tcp client and server test impls at each
other for a loopback server/client test, like how its done in uv::ll

once ipv6 parse/format lands, i can add another test using the entirely
same codebase, but substituting an ip_addr ipv6 varient for the ipv4
varient used in the existing code

still need some other plumbing to get the client/server tests to work
together.

still need implementation for parsing/output formatting and (perhaps?)
representation (for now, i just followef the ipv4 variant's lead and
am representing it as a tuple of 8x u16).

parsing an ipv6 addr is way more complex than parsing an ipv4 addr, so
i'm putting off an implementation here, for now.

candidate solutions:
- could use getaddrinfo() (exists on both POSIX and windows), but with
incompatible fn signatures.
- libuv has a way to parse an ipv6 string into
a sockaddr_in6, but it also requires a port, so it's probably not aprop
for ip_addr

more flexibility..

they're changed into a net::tcp::tcp_err_data record, for now. once the
scope of possible tcp errors, from libuv, is established ill create an
err type for each one and return those where they might occur

* tweaked the layout of sockaddr_in6 struct in anticipation of future use
* changed several uv:ll fn signatures to use generics and be more flexible
with ptr types they get passed
* add uv_err_data and a help fn to return it.. packages up err_name and
err_msg info from uv_get_last_error() stuff..

This should go in libstd, but currently resolve bugs make this not
work.

Send is no longer a subkind of copy. This allows for sendable, but non-copyable resources. Closes #2420.

The old way was inconsistent---the head was unboxed but the
tail was boxed.  This resulted in numerous needless copies and
also made the borrow check unhappy, because the head tended to be
stored in mutable memory.

also: add a non-operator-overloaded method for [] to work around #2378

Closes #2227.

Changed a while loop into a for loop in std::bitv::equal. Yay!