"=>" is not indented as "->" nor "::"

Hi,

"=> is not indented as "->" in function type signature and there is no obvious way to achieve this
through customize-variable.

Expected unified indentation
```
foo
:: (Monad m)
=> Int
-> m String
```

Actual indentation
```
foo
:: (Monad m)
=> Int
-> m String
```

No file or test is named. Start by locating the indentation logic for Haskell function type signatures and the customization path for indentation variables; reproduce the shown example and make `=>` align with `::` and `->`, then verify the existing indentation tests if available.

Written by the indexing model from the issue text.