11-interfaces slides should be split up

[stellentus]

In my opinion, "Satisfying interfaces" should be 3 slides:

• Simple with its ToString
• Complex with its ToString
• A duplicate of the Complex slide (for context), along with Composition (but with Composition in bold)

I think that "Using Interfaces" should be two slides.

• Create a Simple and print it. Avoiding []Stringer will make this easier to understand.
• Then, the slide that already exists, showing that they are all stringers.