Add presentation introduction

[yfyf]

80% of our work is done now.

[yfyf]

FYI I'm not very seriously suggesting to merge this.

[knuton]

An arrow is more like a bento box, I believe!

[yfyf]

I don't see how a bento box models I/O. A bento box is just a container?
A taco, however:

• you can stack foods, just like arrows (via first / second)
• you have unstructured input (it is not all closed up like the burrito -- just slap some cheese on the top!)
• you can consume it left-to-right

I think taco is surely correct.

[knuton]

Clearly the bento box is a better model:

• supports stacking in a much cleaner way: just put two smaller bento boxes in a larger box
• structured input