Add Builder Method on Expressions + Update Proc Macro

[MinisculeGirraffe]

Fixes #19.

Adds a method to handle adding a new expression while keeping the variables in-sync with the other methods

I also threw #12 in here as well.

I can break them up into separate PR's if you'd like.

Fact check me on the proc macro pls. This is like my second time writing one.

I tested everything locally and it seemed to work just fine. It worked both with a manually defined struct, and with one generated by struct-patch.

[neoeinstein]

Thanks for this. I'll be a bit slow on review due to other commitments, but should be able to get an initial review in the next couple of days.