Fix interface hashes including time stamp of dependent files.

Fixes #8144.

Before, the modification time of e.g. #included files (and everything
that ends up as a UsageFile, e.g. via addDependentFile) was taken as
input for the interface hash of a module.

This lead to different hashes for identical inputs on every compilation.

We now use file content hashes instead.

This changes the interface file format.
You will get "Binary.get(Usage): 50" when you try to do an incremental
using .hi files that were created with a GHC 7.7 (only) older than this commit.

To calculate the md5 hash (`Fingerprint`) of a file in constant space,
there now is GHC.Fingerprint.getFileHash, and a fallback version
for older GHCs that needs to load the file into memory completely
(only used when compiling stage1 with an older GHC).