Support decoding from a Get monad

[ezyang]

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

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.

[ezyang]

There's some other related refactoring that could be done, but this is the most important one for me.

[andreasabel]

Is there a reason not to merge this?