Add a section of handling still-too-big wasm

[alexcrichton]

Comparisons of WebAssembly in terms of code size tend to only take a look at the size of the wasm file itself and stop there. In terms of benchmarking though code size is rarely the only factor to consider! This commit adds a section to the code size section of the book to list a few other factors which are important to consider when profiling and measuring code size of wasm files.

[alexcrichton]

An excellent idea, added!