Object inlining and instrumentation for RACCompoundDisposable

`RACCompoundDisposable` sees extremely heavy use in RAC, so it's typically one of the biggest sources of memory usage from within the framework. _(Though it's worth noting here that the memory used by RAC is almost always dwarfed by the application itself.)_

Turns out the internal `CFArray` is somewhat wasteful when there are only one or two entries, and a not-insignificant amount of time is spent within `CFArray` functions.

This PR adds a C array of 2 `RACDisposable`s directly into every `RACCompoundDisposable` instance, so it's always preallocated and quicker to access. I found this to perform better than inlining 1 or 0 disposables, though I didn't try 3 (mostly because I don't want to increase instance size that much).

I saved some Instruments traces, but they're super huge, so numbers will have to do. :trollface:

When testing GitHub for Mac for ~45 seconds:

 * Inlining zero disposables (basically the current `master` behavior) resulted in 2.03 - 3.55 MB of memory occupied solely by `RACCompoundDisposable` and its arrays
 * Inlining one disposable resulted in 2.08 - 2.45 MB used
 * Inlining two disposables resulted in 1.16 - 1.58 MB used

Naturally, there was some variance in the testing (since this is a real world app and it's impossible to perfectly replicate the conditions each time), but those numbers are significant even after that's taken into account.