Refactor API so that keys are generic

[ammario]

This is a substantial refactor of the API, removing the "Embedding" concept and instead centering the API around:

type Vector = []float32

// Node is a node in the graph.
type Node[K cmp.Ordered] struct {
	ID  K
	Vec Vector
}

I believe this API is overall better because:

• The de-virtualization of nodes leads to modest performance improvements
• Library users don't have to declare a custom "Embeddable" type
• There remains only one generic parameter, keeping instantiation simple

[josharian]

API looks nice and simple. Might be nice to say something about how the ordering property of the ID gets used, so folks can reason about it. (I originally assumed it'd be a comparable.) I am heads down on other stuff and then camping, so can't actually try it out until mid- to late-June, I'm afraid...

[ammario]

Might be nice to say something about how the ordering property of the ID gets used, so folks can reason about it.

Will do

I am heads down on other stuff and then camping, so can't actually try it out until mid- to late-June, I'm afraid...

enjoy the the time away!