Preliminary bindist rule (#555)

* Preliminary bindist rule

For now, we only ship `<build root>/{bin, lib}` and the few make build system
related files that are needed to support a simple

```
./configure [--prefix=PATH] && make install
```

workflow. The current binary distributions of GHC support a wider range
of parameters, but I figured it would be a good thing to start with this
and enhance it as we all see fit and perhaps using feedback from GHC HQ
(@bgamari in particular) and bindist users.

* document binary distribution rule in README

* sdist-ghc -> source-dist, Rules.Bindist -> Rules.BinaryDist