Single-line let declarations (and top-level declarations)

Should single-line let declaractions (and possibly also top-level declaractions) be allowed? If so, what should the rules related to that be?

- One proposal from https://github.com/avh4/elm-format/issues/507#issuecomment-426995482 is to only allow it for declarations that have no arguments

Start by reviewing the linked elm-format proposal and the existing rules for single-line let declarations and top-level declarations. Determine whether declarations without arguments should be allowed and define the formatting rules for the accepted cases; done means the project has an agreed behavior and corresponding implementation scope.

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