Use consistent naming convention for type aliases

[lw]

We have a lot of shorthand using alias = long complicated type. One chief example are the types of callbacks. The issue is that we've been inconsistent with the name of the alias: half of them use a snake_case style (e.g., read_callback_fn), whereas others use a CamelCase style (e.g., TReadCallback). There's also the fact that one has a trailing _fn whereas the other one doesn't (we should drop it). And finally there's the fact that the leading T typically denotes a template type parameter, and thus is misleading in aliases. Hence the convention we should converge on is probably just ReadCallback.