Misc cleanup to representing types and generic parameter defs section

[BoxyUwU]

Some small stuff: flattens some very nested chapters into only one layer of nesting, replace some old terminology with new terminolgoy, add a note to the early/late bound chapters warning ppl that they talk about some advanced stuff that was not yet covered. also removes the section on representing constants since it really doesn't go into much detail at all, is somewhat out of date, and talks a lot about generic_const_exprs stuff which isn't super important to talk about

[jieyouxu]

Thanks!