Global debug flag, better error messages

[ZoteTheMighty]

Adds messages to all asserts. Also adds a convenience method for adjusting assert messages when asserts should only signify internal bugs.

Additionally, adds checks to a global debug flag called _G.ROACT_DEBUG, which will toggle whether or not we do asserts in our most performance sensitive code paths.

Repurposes Config to introduce flagged asserts. Uses mutation internally in Config so that consumers can grab the configuration at require time and refer to updated values after they get set.

So far, speed gains are fairly noticeable in most cases:

                             NewReconciler     NewReconcilerOpt  
  fancy (x2000)              1.8738s           1.7932s (-4.3%)   
  hello (x100000)            1.2871s           1.1330s (-12.0%)  
  lifecycle (x100000)        0.8759s           0.6985s (-20.2%)  
  nested-mount (x100000)     5.4379s           4.1647s (-23.4%)  
  nested-update (x100000)    4.3488s           3.5283s (-18.9%)  
  stateful-mount (x100000)   1.4395s           0.9646s (-33.0%)  
  stateful-update (x100000)  0.8171s           0.4630s (-43.3%)  
  update (x100000)           0.7654s           0.7499s (-2.0%)   

Checklist before submitting:

• Added entry to CHANGELOG.md
• Added/updated relevant tests
• Added/updated documentation

[ZoteTheMighty]

I dunno if this is the approach we want to align on, it seems... okay, but I wonder if it's just a little too messy/intrusive?

[ZoteTheMighty]

Some thoughts on this:

• First of all, I should probably remove/clean up GlobalConfig in this same commit. We'll probably make it no-op and issue a warning when interacted with.
• There's one debug flag that controls all features: prop validate, argument type checks of both internal and external apis, and element tracing. It probably makes sense to break this up into separate buckets

[LPGhatguy]

I opened rojo-rbx/rojo#160 for the tooling side implications of changes like this that we might be able to port and take advantage of.

[LPGhatguy]

LGTM