Switch grammar of Natural and Integer

[Gabriella439]

... as standardized in dhall-lang/dhall-lang#141

Now Naturals no longer require a leading + and non-negative Integers
require a leading +

[joneshf]

🎉 🎉 🎉

It's pretty impressive how much of this diff is not source code changes.

The changes look good. But I think some things were missed in comments/docs. These are the ones I found:

• dhall-haskell/src/Dhall.hs

  Line 136 in 6beba69

  >>> input integer "2"

• dhall-haskell/src/Dhall.hs

  Line 238 in 6beba69

  > │ 1 : Integer │ ❰1❱ is an expression that has type ❰Integer❱, so the type

• dhall-haskell/src/Dhall.hs

  Line 243 in 6beba69

  > │ Natural/even +2 : Bool │ ❰Natural/even +2❱ has type ❰Bool❱, so the type

• dhall-haskell/src/Dhall.hs

  Line 365 in 6beba69

  >>> input natural "+42"

• dhall-haskell/src/Dhall.hs

  Lines 378 to 379 in 6beba69

  	>>> input integer "42"
  	42

• dhall-haskell/src/Dhall.hs

  Lines 433 to 434 in 6beba69

  	>>> input (maybe integer) "[1] : Optional Integer"
  	Just 1

• dhall-haskell/src/Dhall.hs

  Lines 446 to 447 in 6beba69

  	>>> input (sequence integer) "[1, 2, 3]"
  	[1,2,3]

• dhall-haskell/src/Dhall.hs

  Lines 459 to 460 in 6beba69

  	>>> input (list integer) "[1, 2, 3]"
  	[1,2,3]

• dhall-haskell/src/Dhall.hs

  Lines 467 to 468 in 6beba69

  	>>> input (vector integer) "[1, 2, 3]"
  	[1,2,3]

• dhall-haskell/src/Dhall.hs

  Line 498 in 6beba69

  >>> input (pair natural bool) "{ _1 = +42, _2 = False }"

• dhall-haskell/src/Dhall.hs

  Lines 520 to 521 in 6beba69

  	>>> input auto "[1, 2, 3]" :: IO (Vector Integer)
  	[1,2,3]

• dhall-haskell/src/Dhall.hs

  Line 1080 in 6beba69

  > +289

• dhall-haskell/src/Dhall/Core.hs

  Line 250 in 6beba69

  -- | > NaturalLit n ~ +n

• dhall-haskell/src/Dhall/Core.hs

  Line 272 in 6beba69

  -- | > IntegerLit n ~ n

  Unsure if that should change or not

I didn't really look through the tests too deeply. I figure the tests have less documentation and more code that will be executed; they ought to catch mismatches.

[Gabriella439]

Good catch! I will fix those and also see if I can add doctests to check those