v4.0.0 (#494)

Features:

* Exception effects / raises for async procedures helping you write more
efficient leak-free code
* Cross-thread notification mechanism for suitable building channels,
queues and other multithreaded primitives
* Async process I/O
* IPv6 dual stack support
* HTTP middleware support alloing multiple services to share a single
http server
* A new [documentation web
site](https://status-im.github.io/nim-chronos/) covering the basics,
with several simple examples for getting started
* Implicit returns, support for `results.?` and other conveniences
* Rate limiter
* Revamped cancellation support with more control over the cancellation
process
* Efficiency improvements with `lent` and `sink`

See the [porting](https://status-im.github.io/nim-chronos/porting.html)
guides for porting code from earlier chronos releases (as well as
asyncdispatch)

chronos.nimble

@@ -1,13 +1,13 @@
 mode = ScriptMode.Verbose
 
 packageName   = "chronos"
-version       = "3.2.0"
+version       = "4.0.0"
 author        = "Status Research & Development GmbH"
 description   = "Networking framework with async/await support"
 license       = "MIT or Apache License 2.0"
 skipDirs      = @["tests"]
 
-requires "nim >= 1.6.0",
+requires "nim >= 1.6.16",
          "results",
          "stew",
          "bearssl",