How to use `fast-tags` instead of `hasktags`?

Thanks for writing and maintaining this Emacs mode! It works really well so far :+1:

I have some difficulties figuring out how to use `fast-tags` instead of `hasktags`. I've installed `fast-tags` and I can generate proper `TAGS` file using the following command from the package root:

```
fast-tags -R -e .
```

However, I don't know how to call it automatically on file save. I tried the following (similar to #1576):

```elisp
(setq
haskell-tags-on-save t
haskell-hasktags-path "~/.cabal/bin/fast-tags"
)
```

But this doesn't generate `TAGS` file on file save. Could you please help to find proper configuration to make `fast-tags` work properly with `haskell-mode`? Thanks in advance!

Start by comparing haskell-tags-on-save and haskell-hasktags-path with the configuration discussed in issue #1576, then inspect how saving a file invokes the configured tag generator. Done means the documented configuration makes saving a Haskell file create or update a TAGS file with fast-tags.

Written by the indexing model from the issue text.