`@arithmetic_rule`

[shashi]

Have a macro

@arithmetic_rule $(D(r)) * ~x => ...

such that it matches,

D(r) * a * b with ~x as a*b (D(r) * b) / a with ~x as b/a

Also similarly enable: D(x) * ~y + ~x

This should simplify writing redundant rules in MethodOfLines.jl