Investigate Perf for FastCount<T> in CollectionExtensions

[RealDotNetDave]

Look into why FastCount is slower than Count() in CollectionExtensions.cs shown in the latest benchmark tests.

[jamescurran]

It appears all the FastCount does is, for List<>s, replace the foreach (that's in the System.Core's Count()) with a for().
The problem is in the line if (predicate(list[listCount])). For that, the compiler adds a bounds-check on the indexed access. The foreach doesn't need that, since the compiler knows the value is within bounds.
(Had list been an array instead of a List<>, the compiler can recognize the for(int i =0; i < array.Length; ++i) idiom, and will remove the bounds check, making it a very tiny bit faster than the foreach)