Adding start of abstract_factory.rs
Adding start of abstract_factory.rs

There are still some errors with it regarding the generics, but I'll
deoxidize them soon. (Heh heh. See what I did there?)

beni55/rust-design-patterns,jdavis/rust-design-patterns,jdavis/rust-design-patterns,jdavis/rust-design-patterns,ianlet/rust-design-patterns,ianlet/rust-design-patterns,ianlet/rust-design-patterns,beni55/rust-design-patterns,jdavis/rust-design-patterns,beni55/rust-design-patterns