[cleanup] Stop using SizeOf, use foo_t<T> and foo_v<T>

This collects some trivial cleanups to simplify and unify code:

• Use sizeof...(T) instead of (the hopefully soon deprecated) SizeOf<T...>::value
• Use the more readable foo_t<T> instead of typename foo<T>::type for standard type traits.
• Use the more readable foo_v<T> instead of foo<T>::value for standard type traits.

In all three cases the nicer version is already used in several places. This MR just makes this consistent.