Teach Forward Mode about Effects and Zeros

[oxinabox]

In forwards mode, if you can prove code is effect free, then it is safe to not execute it if you are just pushing forward zeros & you don't need the primal result.
There might be some other things we can do with effects here too.

Probably worth cross-referencing what is happening in Dex-land with effects.

[oxinabox]

Related: #109

[oscardssmith]

what's the relation?

[oxinabox]

I still don't properly understand #109 but one of the cases Keno thinks it is illegal is if it is used on code with side-effects.