Shake v0.18

[Neil Mitchell]

Hi all,

I've just released Shake 0.18. The big change is that need foo >> need
bar now builds foo and bar in parallel. See the blog post for details:
https://neilmitchell.blogspot.com/2019/05/shake-with-applicative-parallelism.html

This 0.18 bump has no API changes, only runtime behaviour changes, so
I thought I'd play it safe and bump by 0.1. Recent changes which are
relevant include:

* Make files copied to the shared cache read-only. Avoids potentially
disastrous mutation of files. However, your files will be read only,
so functions like removeDirectoryRecursive won't work. I suggest you
use removePathForcibly instead.
* Use parallelism for Applicative adjacent needs - as described in the
blog post.
Support GHC 8.8
* Merge successive Cwd declarations to cmd - so cmd (Cwd a) (Cwd b) is
equivalent to cmd (Cwd (a </> b)).

As always the full changelog is at
https://github.com/ndmitchell/shake/blob/master/CHANGES.txt

Thanks, Neil