Mecca available on macOS

[Jacob Carlborg]

I would like to announce that I've ported Mecca [1] to macOS. It was 
merged in May but I never bothered to announce it until now. Everything 
except the file watcher has been ported and is working. Unfortunately 
the unit tests fail in Travis CI, but they pass locally and in Circle 
CI. Something related to a timer that is failing.

When it comes to the implementation, it's structured so that the new 
code will work on any Darwin platform (it's only tested on macOS). In 
this process I also moved some platform specific code for Linux to the 
`mecca.platform` and `mecca.reactor.platform` packages. This means that 
all platform specific code lives in these two packages and there are no 
version blocks outside of these packages (as far as I could find). The 
version blocks that exist are very few, due the the platform specific 
code lives in their own files. They're just needed to import the correct 
platform specific file. I think this turned out really nice compared to 
the mess that's in druntime, where most files are littered with version 
blocks.

The Darwin implementation uses kqueue, which is available on other 
platforms as well, like the various BSD flavors. The kqueue specific 
code is guarded with its own version block to make it possible to reuse 
on other platforms, if someone wants to port Mecca to a new platform. 
Some timer related code is implemented using libdispatch on Darwin, but 
I think this could be implemented using kqueue as well. I think 
libdispatch is available on *BSD as well, but might not be installed by 
default.

[1] https://github.com/weka-io/mecca

-- 
/Jacob Carlborg