CHANGELOG: `hl.Struct` now has a correct and useful implementation of pprint.

For structs with keys that were not identifiers, we produced incorrect `repr` output. For `pprint`,
we just `pprint`'ed a dictionary (so you cannot even tell that the object was an `hl.Struct`). This
PR:

1. Fixes `hl.Struct.__str__` to use the kwargs or dictionary representation based on whether the
   keys are Python identifiers.
2. Teaches `StructPrettyPrinter` to first try to `repr` the struct (this is what the default pretty
   printer does)
3. Teaches `StructPrettyPrinter` to properly pretty print a struct as an `hl.Struct` preferring the
   kwarg representation when appropriate.
4. Teaches `_same` to use pretty printing when showing differing records.