Fix usages of `unsafe` related to hard-to-model lifetimes

[cswinter]

Various parts of the code use mem::transmute to make lifetimes work out for data referencing strings. More details on the issue here: https://clemenswinter.com/2018/07/09/how-to-analyze-billions-of-records-per-second-on-a-single-desktop-pc/4/ Probably not worth revisiting this until generic associated types are available.