rewriting

Per discussion in #2743 this class is mis-named.

Also, rename LinkStreamValue to just plain LinkStream.

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)) 

Constant operators (i.e. those with arity 0) sometimes have special status as compared with non-constant operators. It would be nice to be able to ask the signature to provide a list of the constants (or the non-constants).