API for multi-arg hvp!/hvp

Right now, it’s not easy (impossible?) to use annotations like `Const` with `hvp!`.

Use case:

Suppose we have a function:

```julia
f(x, y, z) = sum(map(sin, x)) + sum(map(cos, y)) + sum(map(sin, z))
```

It would be nice to support calls like:
```Julia
hvp(
f,
[1.0, 2.0],
Const([1.0, 2.0]),
[1.0, 2.0],
[1.0, 1.0, 1.0, 1.0],
)
```
Open question: how should directions be passed?

If hvp takes a tuple of arguments, should it also take a tuple of direction vectors, or is a single “flattened” direction vector of the correct total dimension sufficient?

Interface options

Option A: one direction per active argument
• For one active argument: `hvp(f, x, Const(y), vx)`
• For two active arguments: `hvp(f, x, y, vx, vy)`

Option B: a single combined direction vector

Pass a single container holding all directions (for example, a vector/tuple of vectors): `hvp(f, x, y, [vx, vy])`

The same question applies to output buffers.

Whichever convention we pick for direction vectors should likely also apply to the output buffers (e.g., one buffer per active argument vs. one combined buffer).

I guess someone can propose alternative options.