Improve Prelude.JSON.render output

[Gabriella439]

Now that we have language support for Text/replace we don't need
to use Text/show any longer. In particular, one of the limitations
of Text/show was that we were using it both to render valid Dhall
values and also to render valid JSON strings, which constrained it
to produce weird output for $ (\u0024). However, now that we
have Text/replace we can use a JSON-specific escaping strategy.

This also implies that we can make two further improvements (not
included in this pull request):

• We could standardize the behavior of Text/show in terms of
  Text/replace

  … or even remove Text/show as a built-in after deprecation.

• We can now improve the behavior of Text/show to escape $ as
  \$ instead of \u0024

  … now that it's no longer used for escaping JSON strings.

[sjakobi]

I was curious how this would affect the size of the binary encoded Prelude:

$ echo https://raw.githubusercontent.com/dhall-lang/dhall-lang/master/Prelude/package.dhall | dhall --alpha | dhall encode | wc
      3    1043   93241
$ echo https://raw.githubusercontent.com/dhall-lang/dhall-lang/22bb26a72e727977ad0a1a17a8e7652328a80170/Prelude/package.dhall | dhall --alpha | dhall encode | wc
     59    1267  102105

So the cache size is about 9.5% bigger now, which doesn't seem too bad.

[SiriusStarr]

... or even remove `Text/show` as a built-in after deprecation.

I would be fine with removing Text/show so long as text escaping ends up in the Prelude. I think it's important to have a quick and easy Text.escapeFor.Dhall and/or Text.escapeFor.Json or whatnot, because I currently can just slap Text/show on the problem and assume it works rather than having to remember every single thing that needs to be escaped for Json and invariably forgetting one.