Send textual messages to a Handle in a thread-friendly way.

The motivation for this package is described in a blog post on Haskell's
Missing Concurrency Basics
(http://www.snoyman.com/blog/2016/11/haskells-missing-concurrency-basics). The
simple explanation is, when writing a line of textual data to a Handle -
such as sending some messages t o ther terminal - we'd like to have the
following properties:

* Properly handle character encoding settings on the Handle
* For reasonably sized messages, ensure that the entire message is written
  in one chunk to avoid interleaving data with other threads
  * This includes the trailing newline character
* Avoid unnecessary memory allocations and copies
* Minimize locking
* Provide a simple API