Switch order of current core exercises (#800)

As part of [The Track Anatomy Project][1], the Haskell core track is
restructured in #798 using @F3PiX'es multi-stage framework referred to
as "The Track Tool".

Since The Track Tool is still under development, it isn't documented
yet. But the execution of the first step can be viewed for other
languages, e.g. exercism/javascript#590.

---

Proposal:

 - Switch "Bob" and "Pangram".

   They're both about handling textual data.

   Pangram has fewer gotchas, is quicker done, has more variations among
   its alternative solutions, and doesn't lend itself as well to `Data.Text`.
   (See #795.) Bob often involves more refactoring, has more learning
   objectives and has the extra "You could do it with Data.Text, too!" part.

   This way people don't try to solve Pangram with Data.Text which is possible
   but not ideal without mono-traversable, and they get an exercise more under
   their belt before being thrown into a lot of refactoring.

 - Switch "Nucleotide Count" and "Sum of Multiples".

   Eventually I think we should delete "Sum of Multiples" and "Grains" (see #761).

   But since The Track Tool does not address removal yet, I want to make
   "Nucleotide Count" come immediately after "RNA Transcription", since
   they're both thematically related and Nucleotide Count uses Either
   while RNA Transcription uses Maybe for monadic error handling, so they
   practice the same thing in progressive difficulty.

The proposed track reordering looks like:

1. hello-world
2. leap
3. space-age
4. **pangram**
5. **bob**
6. collatz-conjecture
7. rna-transcription
8. **nucleotide-count**
9. sum-of-multiples
10. grains

[1]: https://exercism.io/blog/track-anatomy-project

config.json

@@ -37,8 +37,8 @@
       ]
     },
     {
-      "slug": "bob",
-      "uuid": "e570c730-12e5-4b9e-adb0-eae85360e1b5",
+      "slug": "pangram",
+      "uuid": "42cb6880-0c4b-4455-815b-aac94c05db18",
       "core": true,
       "unlocked_by": null,
       "difficulty": 1,
@@ -47,8 +47,8 @@
       ]
     },
     {
-      "slug": "pangram",
-      "uuid": "42cb6880-0c4b-4455-815b-aac94c05db18",
+      "slug": "bob",
+      "uuid": "e570c730-12e5-4b9e-adb0-eae85360e1b5",
       "core": true,
       "unlocked_by": null,
       "difficulty": 1,
@@ -79,24 +79,24 @@
       ]
     },
     {
-      "slug": "sum-of-multiples",
-      "uuid": "e125a32e-60ca-4e59-ae35-01f3b1907ccd",
+      "slug": "nucleotide-count",
+      "uuid": "00feb0e9-ae1e-43fc-a0ea-8f2d0d0b9f49",
       "core": true,
       "unlocked_by": null,
-      "difficulty": 1,
+      "difficulty": 2,
       "topics": [
-        "math",
-        "number_theory"
+        "either"
       ]
     },
     {
-      "slug": "nucleotide-count",
-      "uuid": "00feb0e9-ae1e-43fc-a0ea-8f2d0d0b9f49",
+      "slug": "sum-of-multiples",
+      "uuid": "e125a32e-60ca-4e59-ae35-01f3b1907ccd",
       "core": true,
       "unlocked_by": null,
-      "difficulty": 2,
+      "difficulty": 1,
       "topics": [
-        "either"
+        "math",
+        "number_theory"
       ]
     },
     {