book: document input relations, naive rules, location-insensitive rules

[lqd]

This PR starts describing the input relations "pre-liveness". More details about the liveness relations and atoms will be coming later. Also describes two of the three variants in datalog form.

Represents the state of things after #156 with the naming and rules from our rules hackmd:

• added documentation about the input relations (with the new relations used in subset errors)
• added documentation about the Naive rules (without the placeholder loans from the hackmd)
• added documentation about the LocationInsensitive rules
• some summary work about the rules and pipelines
• some links to blog posts and the polonius talk at Rust Belt Rust

r? @nikomatsakis

[lqd]

Also note that the syntax highlighter that comes with mdbook does not by default enable support for the prolog code blocks that I've used for the soufflé datalog rules: the md files will look a tiny bit better on github than they will in the deployed site (unless we also override the syntax highlighter to embed a version of the highlighter that enables prolog highlighting)

[nikomatsakis]

I've skimmed this and it looks quite good. I'm inclined to merge it and give it more detailed reads later over time.