Improve Dictionary<K,T> CQ for primitive BCL ValueTypes

Incorporate @AndyAyersMS [Potential CQ improvements in Dictionary<K,T>](https://github.com/dotnet/coreclr/compare/master...AndyAyersMS:DictionaryHacks?expand=1)

Specialize Equals and KeyHashCode to check for each of the BCL ValueType primitives, cast via object to the type and use them directly.

Types are `byte`, `sbyte`, `ushort`, `short`, `uint`, `int`, `ulong`, `long`, `IntPtr`, `UIntPtr`, `Guid`

Split functions between default comparer and custom comparer