Reduce unnecessary DOM rendering from renderEach() in prelude.kt

[sanity]

Create an extension function for rendering Collections within KVars:

val c : KVar<List<Int>> = KVar(listOf(1, 2, 3))
// inside Kweb {}
renderEach(c) { v : KVar<Int> ->
  h1().text(v.map { it.toString() })
}

The trick here will be to efficiently handle element insertion and deletion while minimizing DOM modifications.

see also: https://github.com/kwebio/kweb-core/issues/123#issuecomment-622941084

[sanity]

Simple implementation of this here released in 0.7.12, although unnecessarily re-renders list if the list size changes so leaving this issue open until that can be made more efficient.

[github-actions[bot]]

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