Implement applyl_at and applyr_at functions

[redbaron]

It adds applyl_at and applyr_at functions which apply single argument function to the nth HList element. It is implemented as a fold function which carries target and current depth levels as well as function application result in the accumulator.

Because n can be outside of the hlist length, function returns Option<_> with None returned if n is larger than list size.

It also hasapply*_args_at variants which adds args param to the accumulator to be passed to the function as a poor replacement for missing self (and therefore closure) param.

This is a draft PR to collect early feedback, more specifically:

• is it something you'd like to see in this crate?
• should it be standalone function like it is now or you prefer it to be a method on the HCons?
• functions/arguments naming suggestions.

[redbaron]

Added apply*_args_at variants

[lloydmeta]

Out of curiosity, what is the use case for this?

In general, I'd like to avoid failable-at-runtime things in this crate because it feels like we could avoid them since we (should) know almost everything at compile time. For instance, we could use a Labelled Hlist, pluck out the element we want by type (we'd get a compile time error if it does not exist or is duplicated), then apply a function?

[redbaron]

Not everyting is known at compile time. In my experiments I have a "query plan" (list of operations) in the HList and need to execute them in order when right conditions arise, hence the runtime n param to pick one to run.