Invoking+Awaiting

[jnyrup]

This fixes #872 and complements #1015 by implementing

Func<TResult> Invoking<T, TResult>(this T subject, Func<T, TResult> action)

For consistency this also implements

Func<Task<TResult>> Awaiting<T, TResult>(this T subject, Func<T, Task<TResult>> action)

though this currently doesn't have much benefit as both Func<Task> and Func<Task<T>> uses AsyncFunctionAssertions.