Define macros so that we can easily use Crosslink notation in the book

[daira]

This is separate from #85, and does not block it, but would simplify maintenance of the Crosslink content and improve notational consistency.

[nathan-at-least]

It looks like the macro system used in Halo book is from mdbook-katex for reference.

[nathan-at-least]

My Desiderata:

• We can define macros for any kind of term. For example, I imagine categories of terms, such as "consensus rule", "mathematical variable", etc…
• We can use macros for every reference to the defined term.
• Unknown macro expansions cause rendering errors (e.g. if we misspell a reference due to a typo).
• We can refer to these terms inline outside of latex-style formula. (This seems feasible just by inlining formulae, eg md context could say The $\ExtensionRule$ ensures that ….
• Each reference expands to a hyperlink to a definition. (<- this is the kind of feature that really helps me, since I often need to review definitions as I'm reading.)

[nathan-at-least]

@daira found that MathJax supports inline hyperlinks. The trick here is how can we ensure the link referent is coherent with mdbook link definitions (e.g. ../terminology.md#definition-extension-rule gets translated to an HTML destination).