Move method definitions inside classes

[hollasch]

We currently define some class methods inside classes, and some outside. Given our code structure (#include all source files from a single main-defining file), this would help keep things clean and consistent.

Some have argued that we should separate method definitions from headers, and I would certainly do that for my own implementation, but that wouldn't buy anything for our expository purposes, and would roughly double the number of source files that people would have to follow.

Currently, book 1 is a little over 1200 lines total, and books 2 & 3 about 2300 lines each. This simplicity serves us and our readers well.

[hollasch]

Done.