!feat: change zip underlying implementation

[JaapWijnen]

This simplifies the protocol requirements for DifferentiableSequence but breaks the previous api (people shouldn't have adopted the protocol but it can't be kept internal). The change enables us to conform StencilCollection to DifferentiableCollection.

[tmcdonell]

breaks the previous api (people shouldn't have adopted the protocol but it can't be kept internal)

You mean the change from appendContribution(of:) to writeTangentContribution(of:at:)? I would also consider that an internal/private interface that is not meant to be used by people, so I think that's fine to change.

Nit: switch the order of the arguments to match initializeElement(at:to:)? And is the consuming annotation applicable to add here as well? (not sure, honest question)

[JaapWijnen]

switch the order of the arguments to match initializeElement(at:to:)

Oh great suggestion!

And is the consuming annotation applicable to add here as well? (not sure, honest question)

I'm not sure what you're referencing here exactly. Could you elaborate?

[tmcdonell]

I'm not sure what you're referencing here exactly. Could you elaborate?

The type signature for initializeElement(at:to:) has this consuming annotation; not sure what that does but was wondering if it's useful for us here as well.

func initializeElement(
    at index: UnsafeMutableBufferPointer<Element>.Index,
    to value: consuming Element
)