bootstrap.sh: fix transformers version regex

mtl-2.2.1 requires transformers ==0.4.*.
Merge pull request #2268 from tuncer/mtl-transformers-requirement

bootstrap.sh: fix transformers version regex
Add GHCJS support to the Simple build infrastructure

This also restructures some of the GHC, HcPkg and Haddock modules
to make it easier to share the code between multiple GHC-derived
compilers.