moving to using the GADT

The problem this exposes has to do with requires. As implemented,
requires yields either a Property HasInfo or a Property NoInfo depending
on its inputs. That works. But look what happens when it's used:

*Propellor.Types> let foo = IProperty "foo" (return NoChange) mempty mempty
*Propellor.Types> let bar = IProperty "bar" (return NoChange) mempty mempty
*Propellor.Types> foo `requires` bar

<interactive>:17:5:
    No instance for (Requires (Property HasInfo) (Property HasInfo) r0)
      arising from a use of `requires'
    The type variable `r0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there is a potential instance available:
      instance Requires
                 (Property HasInfo) (Property HasInfo) (Property HasInfo)
        -- Defined at Propellor/Types.hs:167:10
    Possible fix:
      add an instance declaration for
      (Requires (Property HasInfo) (Property HasInfo) r0)
    In the expression: foo `requires` bar
    In an equation for `it': it = foo `requires` bar

This can be avoided by specifying the result type:

*Propellor.Types> (foo `requires` bar) :: Property HasInfo
property "foo"

But then when multiple `requires` are given, the result type has to be
given each time:

*Propellor.Types> (foo `requires` bar `requires` bar) :: Property HasInfo
<interactive>:22:6:
    No instance for (Requires (Property HasInfo) (Property HasInfo) x0)
      arising from a use of `requires'
    The type variable `x0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there is a potential instance available:
      instance Requires
                 (Property HasInfo) (Property HasInfo) (Property HasInfo)
        -- Defined at Propellor/Types.hs:167:10
    Possible fix:
      add an instance declaration for
      (Requires (Property HasInfo) (Property HasInfo) x0)
    In the first argument of `requires', namely `foo `requires` bar'
    In the expression:
        (foo `requires` bar `requires` bar) :: Property HasInfo
    In an equation for `it':
        it = (foo `requires` bar `requires` bar) :: Property HasInfo

<interactive>:22:21:
    No instance for (Requires x0 (Property HasInfo) (Property HasInfo))
      arising from a use of `requires'
    The type variable `x0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Requires
                 (Property NoInfo) (Property HasInfo) (Property HasInfo)
        -- Defined at Propellor/Types.hs:175:10
      instance Requires
                 (Property HasInfo) (Property HasInfo) (Property HasInfo)
        -- Defined at Propellor/Types.hs:167:10
    Possible fix:
      add an instance declaration for
      (Requires x0 (Property HasInfo) (Property HasInfo))
    In the expression:
        (foo `requires` bar `requires` bar) :: Property HasInfo
    In an equation for `it':
        it = (foo `requires` bar `requires` bar) :: Property HasInfo

*Propellor.Types> (((foo `requires` bar) :: Property HasInfo) `requires` bar) :: Property HasInfo
property "foo"

Yuggh!

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