Add (I)TypeActivator.CreateFactory

- The factory returned from `CreateFactory` should be be cached.
- Once generated, the factory is faster than `CreateInstance` since
  there is no need for repeated reflection.

#105 

I tested the performance of `CreateFactory` using a hacked-together microbenchmark that I ran on my workstation. Over a large number of iterations (10 million), I saw about a 4.5x improvement over `CreateInstance` when activating a very simple class:

        // You can add this a test in TypeActivatorTests.cs
        public void TypeActivatorCreateFactoryMicroBenchmark()
        {
            var serviceProvider = new ServiceCollection()
                .AddTransient<IFakeService, FakeService>()
                .AddTransient<ITypeActivator, TypeActivator>()
                .BuildServiceProvider();

            var typeActivator = serviceProvider.GetService<ITypeActivator>();

            var stopWatch = new System.Diagnostics.Stopwatch();
            stopWatch.Start();

            var factory = typeActivator.CreateFactory(typeof(AnotherClassAcceptingData), new[] { typeof(string), typeof(string) });
            for (var i = 0; i < 10000000; i++)
            {
                factory(serviceProvider, new[] { "1", "2" });
            }

            stopWatch.Stop();
            System.Console.WriteLine(stopWatch.Elapsed.TotalSeconds);
        }

Sample output:
    6.7144074
    6.7561712
    6.5328341


Results without compiled expressions (this is a different, slower, implemenation of `ITypeActivator.CreateFactory`):

        // This is the TypeActivator.CreateFactory method without expression building
        public Func<IServiceProvider, object[], object> CreateFactory(Type instanceType, Type[] parameterTypes)
        {
            ConstructorInfo constructor;
            int?[] parameterMap;

            FindApplicableConstructor(instanceType, parameterTypes, out constructor, out parameterMap);

            var ctorParams = constructor.GetParameters().Select(p => p.ParameterType).ToArray();

            return (sp, args) =>
            {
                var argsArray = new object[ctorParams.Length];

                for (var i = 0; i < ctorParams.Length; i++)
                {
                    if (parameterMap[i] != null)
                    {
                        argsArray[i] = args[parameterMap[i].Value];
                    }
                    else
                    {
                        argsArray[i] = sp.GetService(ctorParams[i]);
                    }
                }

                return constructor.Invoke(argsArray);
            };
        }

Sample output (same microbenchmark):
    14.1813902
    12.9400812
    13.0804346


Results for `TypeActivator.CreateInstance`:

        public void TypeActivatorCreateInstanceMicroBenchmark()
        {
            var serviceProvider = new ServiceCollection()
                .AddTransient<IFakeService, FakeService>()
                .AddTransient<ITypeActivator, TypeActivator>()
                .BuildServiceProvider();

            var typeActivator = serviceProvider.GetService<ITypeActivator>();

            var stopWatch = new System.Diagnostics.Stopwatch();
            stopWatch.Start();

            for (var i = 0; i < 10000000; i++)
            {
                typeActivator.CreateInstance(serviceProvider, typeof(AnotherClassAcceptingData), new[] { "1", "2" });
            }

            stopWatch.Stop();
            System.Console.WriteLine(stopWatch.Elapsed.TotalSeconds);
        }

Sample output:
    28.1808019
    31.1729289
    30.7616878



Could you move variables to the top of the class declaration? Also mark it `readonly`