Optimize UnboxValue performance, safety and memory usage

- Avoids unnecessary boxing of the converted values by storing them in
unmanaged memory.
- Wrap ToNative-method in a delegate and cache it
- Fixes returning address to unpinned memory by pinning POD-types for
a short period of time.

Source/Engine/Engine/NativeInterop.Unmanaged.cs

@@ -650,7 +650,6 @@ namespace FlaxEngine.Interop
             if (!type.IsValueType)
                 return ManagedHandle.ToIntPtr(handle);
 
-            // HACK: Get the address of a non-pinned value
             return ValueTypeUnboxer.GetPointer(value, type);
         }