Named rules

We should be able to attach names to rules, so that normalization steps (and error messages?) can be better understood by users. For example:

```
normalize(@term(diff(x + y, x)))
- @term(diff(x,x) + diff(y,x)) by sum rule in differentiation
- @term(one(x) + diff(y, x)) by linear rule of differentiation
- @term(1 + diff(y, x)) by multiplicative identity of a number
- @term(1 + zero(x)) by assumption that variables are independent
- @term(1 + 0) by additive identity of a number
- @term(1) by removal of additive identity
```

Start by locating the normalize entry point and the rule-application code referenced by the example. Determine how rule names could be represented and surfaced in normalization steps or error messages; the issue does not identify files, tests, or a more specific completion criterion.

Written by the indexing model from the issue text.