#42 Conditionally build hot path and a few select large component trees unified.

I observed over at Moonzilla that they have decided to start building things unified again. Some of the reasoning is sound specifically in regards to the JS engine but I think not going the extra mile (and actually doing what Mozilla toyed with) and making those bits that benefit from concatenated sources able to build that way but also keep the ability to build and test non-unified which also tends to help ensure that deprot doesn't creep back in just because it isn't a configuration you personally use.

Over there, of course, the lazy choice was made to just blanket reverse the bulk of work I spent a lot of time on for the JS engine while having a few backhanded comments about it. The rest was an SnR with manual work job.

Just to be clear, we plan to do virtually the same thing except the de-unified sources priority remains and so we will do it in a way that each major tree re-unified can be built either way on that component basis.

So that the previous work is added to and not merely chucked in the bin to go to shit later.

This is technically apart of #41 but since it isn't copypasta or conformed copypasta it needs its own issue to refer to.