Mark up consensus rules in protocol spec

[daira]

The goals are:

• give a name to each rule, which can be correlated with the corresponding code in zcashd and zebrad;
• define an anchor URL for each rule that code can refer to;
• allow easy searching for the code implementing each rule.

See also zcash/zcash#3959 and zcash/zcash#3957.

[defuse]

I like this, once we have an anchor for each consensus rule we can grep the codebases for the anchors to find where all the rules are implemented (and to find any that are not implemented).