Preserve single line function arguments

[brandonchinn178]

Currently, function arguments are forced to be on multiple lines if the whole type is multiline:

foo ::
  Show a =>
  a ->
  String

This allows the arguments to stay on the same line if they were originally:

foo ::
  Show a =>
  a -> String

If the user wrote the function args on multiple lines, it will stay on multiple lines. So this change is not a breaking change for people with already formatted code

ref. https://github.com/fourmolu/fourmolu/pull/277 https://github.com/fourmolu/fourmolu/pull/410