Use Binary to save/restore persistent build config

[ttuegel]

See #2058 and #1841. This is a big pull, but it's mostly just adding deriving Generic and instance Binary a for all the types in LocalBuildInfo. As I noted in #1841,

GHC ships with binary, and since GHC 7.2, binary has a generic instance. GHC 7.2.1 was released 9 August 2011, so it just fell within our 3 year backward-compatible window! That means we can migrate to Binary instead of Read/Show with no boilerplate.

My profiling shows that allocation during a no-op build in the Cabal tree drops from 540 MB to 98 MB, an 82% reduction! The time profile shows that 98.9% of the time for a no-op build is spent in GHC, so I think this is the best we can do to speed things up on the Cabal side. (Maybe we can convince GHC to use binary package databases?) I've tested on GHC 7.8.3, as soon as it passes on Travis, I'll pull this.

[ezyang]

LGTM.

[ttuegel]

Unfortunately, build fails on GHC 7.4 because of some name collision between Prelude and Control.Exception. (That's easy to fix.) And, it's failing on GHC 7.2 because of some issue with Binary and the standalone-derived Generic instance for Version. Apparently, binary isn't as compatible with GHC 7.2 as the docs claim. I'll try an earlier version, but for now this is broken.

[23Skidoo]

Looks good in general. I think that we can drop GHC 7.2 compatibility for the next Cabal release - 7.2.2 was released in November 2011, so by the time GHC 7.10.1/Cabal 1.22 will come out, it will be >3 years old.

[23Skidoo]

Maybe we can convince GHC to use binary package databases?

I believe that GHC already does that.

[ttuegel]

I just checked, and on my system with GHC 7.8.3, /usr/lib/ghc-7.8.3/package.conf.d/*.installedconf are all the products of Show InstalledPackageInfo.

[23Skidoo]

@ttuegel

Interesting, I have no .installedconf files, only .conf:

$ ls ~/.ghc/i386-linux-7.8.2/package.conf.d | ag 'installedconf$' | wc -l
0
$ ls ~/.ghc/i386-linux-7.8.2/package.conf.d | ag 'conf$' | wc -l 
40

And there's also a binary cache file in that directory, package.cache.

[ttuegel]

@23Skidoo I have a package.cache too, but it seems to only pertain to boot libraries. This must be something about how my distro packages things; I'll investigate more there.