Previously, we had generalized the argument kind to `Error` in commit
077aee5 to permit passing things to `Error`
besides `Symbol`s. Back then, there was no way to `show` things at the type
level, nor was there a way to manipulate `Symbol`s in any meaningful fashion,
so this seemed like a reasonable choice.

Nowadays, however, the story is different. There is a type-level `Show_`
function, and the API for manipulating `Symbol`s is nearly as expressive as the
API for manipulating `String`s.  What's more, making `Error`'s argument kind
more general introduces ambiguity-related issues when deriving `Enum` instances
with `OverloadedStrings` enabled, as observed in #89.

In light of this, I have changed the API such that:

* The kind of the argument to `Error` (as well as the related
  `ErrorWithoutStackTrace` function) is now `Symbol`.  In this sense, this patch
  reverts 077aee5.
* There is now a new `Data.Singletons.Base.PolyError` module that provides a
  `PolyError` function. `PolyError` provides a kind-polymorphic `Error`
  interface much like what the previous type of `Error` was, so any existing
  code that relied on the argument of `Error` being kind-polymorphic can be
  migrated over to use `PolyError`.

Resolves #89 (hopefully for good this time).