Make snapshot application communicative

[LPGhatguy]

This change is part of an ongoing effort to make Rojo deterministic, which makes it more robust as well as easier to test and debug.

Without this change, Rojo has nondeterministic child ordering once changes are applied to a running DOM, like in a live sync session. That means that restarting the Rojo server can result in a slightly different tree being presented, which has made snapshot testing a little bit sketchy.

The following things are deterministic with reordered change events either in master or with this change:

• The order of in-memory filesystem nodes
• The order of instances defined in a project
• The order of children in snapshots
• The order of children in RbxTree objects, now with patches

The following things still need to be modified to be deterministic in the face of patches being reordered:

• The order of children in the Roblox DOM in the plugin
  • This is sort of tricky to do well
• The order of properties being applied in the plugin
• The order of properties being serialized in rbxmx, rbxlx, rbxm and rbxl files

TODO:

• Publish new version of rbx_dom_weak that allows reordering of children

[ZoteTheMighty]

Forgot to comment before, but this looks good! Seems like a pretty straightforward way to do what you want.