Fix keys, normalize holes

[dead-claudia]

Description

• Require fragments to either have only keyed children or no keyed children.
• Ban holes from keyed fragments, to reduce complexity in the keyed diff algorithm.
• Normalize booleans, null, and undefined to be a single hole sentinel null, to simplify representation.
• Document a common pattern on how to have a programmatically replaceable node, to resolve some confusion around that.

Motivation and Context

Closes #2434
Fixes #2248

Also resolves a class of paper cuts that have found their way onto Gitter the past several months with v1.

How Has This Been Tested?

Added a couple tests, updated a bunch, rewrote a few.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md

[ancms2600]

moved to: #2526
Feature Request: Normalize vnode.children passed to m() hypertext
(flatten arrays, omit null, undefined and false)

[dead-claudia]

@ancmikesmullin Please file a new issue with that suggestion.