feat: explicit conversion method implementations

[cdbrkfxrpt]

Following a convo with JG, I am planning to:

• aim for Option-like semantics on MaybeMultiple;
• aim for Vec-like semantics on Multiple;
• be very deliberate about the semantics of both.

Specifically, this means that I want methods implemented on MaybeMultiple and Multiple to contribute to the reason why both of them exist, which is to make the semantics of a container that must contain at least two elements - Multiple - and an enumeration which can distinctively contain no elements, one element or multiple elements - MaybeMultiple - explicit in the type system. Implementing convenience conversion functions like From<Vec<T>> on MaybeMultiple, for example, does the opposite of this: it makes the conversion explicit, but in code it will always just be a call to .into() and thus not really visible. Making the call a MaybeMultiple::collapse_vec_into(v) for turning v: Vec into a MaybeMultiple seems a better choice. I hope to find others in the same vein, and I'd of course be thankful for second opinions.