Igniter.Refactors.Rename.rename_function/3 should update the @spec associated with the definition

[zachallaun]

Something I noticed while testing a rename_function call. It will upgrade the function name at the definition site, but not the @spec preceding it, so renaming foo/2 to bar/2 will get you something like:

@spec foo(term(), term()) :: term()
def bar(x, y) do
  ...
end