Fix vector tests warnings about obsolete Marshal.SizeOf

The System.Numerics.Vectors tests were causing build warnings like the following:

Vector2Tests.cs(15,29): warning CS0618: 'System.Runtime.InteropServices.Marshal.SizeOf(System.Type)' is obsolete: 'SizeOf(Type) may be unavailable in future releases. Instead, use SizeOf<T>().

This changes does exactly that.
Merge pull request #221 from stephentoub/fix_vector_test_warnings

Fix vector tests warnings about obsolete Marshal.SizeOf