bfpg-aeson: rearrange the Value slide

Put the Value sum type before the Object and Array type aliases,
which I hope will improve the way I explain things.

Also remove the strictness annotations which are irrelevant,
probably unfamiliar to many and possibly confusing.
bfpg-aeson: typospotting and minor wording changes
bfpg-aeson: remove the SizedBase64Integer slides

The SizedBase64Integer take a while to work through, are heavy in
cryptography-heavy (irrelevant to the topic of JSON serialisation)
and are probably not as interesting as the other two implementation
details being investigated, w.r.t. JSON/aeson.

Therefore, remove these slides.
bfpg-aeson: simplify object parsing slide

Remove the ghci session demonstrating the composition of (.:?) and
(.!=) since it will make the audience go cross-eyed.  Also improve
the appearance of the slide by breaking each function into its own
<pre> block.
bfpg-aeson: general slide improvements

* simplify or elaborate some terminology/points
* line a bunch more things up in code examples
* split some of the <pre> blocks for clarity, and in some cases
  classify them as slides for progressive revelation
* add a point about better error messages in argonaut