Support decoding from a Get monad

I prefer not to ask for convenience functions willy-nilly, but I think refactoring `decodeFileOrFail` with this signature would be useful:

``` haskell
decodeGetFileOrFail :: Get a -> FilePath -> IO (Either (ByteOffset, String) a)
```

The reason is because there is a nontrivial chunk of code for running the incremental parser that preferably we'd avoid duplicating. I'll submit a PR soon.

Start by reading the existing decodeFileOrFail entry point and the incremental parser path it uses. Check the proposed decodeGetFileOrFail signature against the current API; done means decoding from a Get value works without duplicating the nontrivial parser-running logic.

Written by the indexing model from the issue text.