[FEATURE | BREAKING] Change semantics of in-element to match emberjs/rfcs#287

[chadhietala]

This implements the clearing semantics of in-element as it pertains to emberjs/rfcs#287. Instead of appending we need to clear the contents of the destination element unless insertBefore was explicitly set to null.

This is required work for emberjs/rfcs#418

[chancancode]

I think I have a few recommendations:

• Need tests for the "replace" case (i.e. when insertBefore is not specified).
• Option<null> doesn't make sense, since it just expands into null | null.
• I think it would make sense to keep the internal pushRemoteElement API taking an element and a nextSibling. This is basically the "DOM cursor" concept that is used throughout the element builder code. Changing this would make it inconsistent with e.g. pushElement.
• Instead, in the clearing case, I think it would make more sense for the caller to clear the target element (may require adding APIs to the element build or somewhere else, not sure), then call pushRemoteElement with the cleared element and null nextSibling.

[chadhietala]

I added the test. We can do a refactor in the future to break apart the clearing out the target. I had some questions about the Enum map for operations since it has dups in it.