Named rules

[HarrisonGrodin]

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