Fix function Generic's custom instances for sum-types on ligoLayout

Description

It appears that there is a bug in the custom derived Generic instances for sum types using ligoLayout introduced in !731 (merged)

Hint: from the resulting error message it seems that while the constructors are correctly reordered in the Rep type the to and from functions are not and the TH splice fails.

Steps to reproduce

It sufficient to modify the LigoSumType used in Test.Lorentz.CustomValue to be:

data LigoSumType
  = LigoSumTypeB Integer
  | LigoSumTypeA ()

Note: I suggest keeping the () even tho it is unnecessary for IsoValue because otherwise the TH error is much less helpful.

Expected behaviour

The existing test pass in the form described above.

Actual behaviour

The Generic instance from customGeneric fails to type-check.

Environment

• ArchLinux (x64)
• Current master branch