Adding Const Generics section to Rust Book

Hi

I have recently written a new section at the end of chapter 19 covering const generics for the rust book and have just created the pull request for it. In it I have made multiple points about how const generics works and how the compiler works around them. If possible, if you get the chance, please check it out let me know of any errors (especially in regards to my statements concerning how the compiler works), corrections that I need to make or additions you believe would help.

Link to the pull request : link

Note: you will need mdbook to build the book and see all its code examples

Thanks