Remove some boxing from tuples with >= 8 elements

[stephentoub]

Remove some boxing from tuples with >= 8 elements

Take advantage of #14698 to avoid boxing the TRest argument and improve devirtualization.

Method	Toolchain	Mean	Error	Ratio	Allocated
GHC7	Old	2.043 ns	0.0100 ns	1.00	-
GHC7	New	2.042 ns	0.0135 ns	1.00	-
GHC8	Old	8.489 ns	0.0917 ns	1.00	24 B
GHC8	New	3.230 ns	0.0159 ns	0.38	-
GHC9	Old	9.062 ns	0.0459 ns	1.00	24 B
GHC9	New	3.230 ns	0.0194 ns	0.36	-
GHC10	Old	9.644 ns	0.1006 ns	1.00	32 B
GHC10	New	3.479 ns	0.0161 ns	0.36	-
ToString7	Old	214.852 ns	0.4342 ns	1.00	208 B
ToString7	New	209.641 ns	0.5081 ns	0.98	208 B
ToString8	Old	257.0 ns	4.019 ns	1.00	280 B
ToString8	New	242.9 ns	2.952 ns	0.95	256 B
ToString9	Old	268.936 ns	1.0210 ns	1.00	288 B
ToString9	New	266.940 ns	0.9469 ns	0.99	264 B
ToString10	Old	318.626 ns	0.9727 ns	1.00	416 B
ToString10	New	307.865 ns	1.2496 ns	0.97	384 B
Index7	Old	4.067 ns	0.0813 ns	1.00	24 B
Index7	New	3.988 ns	0.0256 ns	0.98	24 B
Index8	Old	7.906 ns	0.0354 ns	1.00	48 B
Index8	New	5.134 ns	0.0505 ns	0.65	24 B
Index9	Old	7.607 ns	0.0418 ns	1.00	48 B
Index9	New	4.996 ns	0.0926 ns	0.66	24 B
Index10	Old	8.989 ns	0.0542 ns	1.00	56 B
Index10	New	5.113 ns	0.0422 ns	0.57	24 B

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Runtime.CompilerServices;

[MemoryDiagnoser]
public class Program
{
    public static void Main(string[] args) => BenchmarkSwitcher.FromTypes(new[] { typeof(Program) }).Run(args);

    private (int, int, int, int, int, int, int) _v7 = (1, 2, 3, 4, 5, 6, 7);
    private (int, int, int, int, int, int, int, int) _v8 = (1, 2, 3, 4, 5, 6, 7, 8);
    private (int, int, int, int, int, int, int, int, int) _v9 = (1, 2, 3, 4, 5, 6, 7, 8, 9);
    private (int, int, int, int, int, int, int, int, int, int) _v10 = (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

    [Benchmark] public int GHC7() => _v7.GetHashCode();
    [Benchmark] public int GHC8() => _v8.GetHashCode();
    [Benchmark] public int GHC9() => _v9.GetHashCode();
    [Benchmark] public int GHC10() => _v10.GetHashCode();

    [Benchmark] public string ToString7() => _v7.ToString();
    [Benchmark] public string ToString8() => _v8.ToString();
    [Benchmark] public string ToString9() => _v9.ToString();
    [Benchmark] public string ToString10() => _v10.ToString();

    [Benchmark] public object Index7() => ((ITuple)_v7)[6];
    [Benchmark] public object Index8() => ((ITuple)_v8)[7];
    [Benchmark] public object Index9() => ((ITuple)_v9)[8];
    [Benchmark] public object Index10() => ((ITuple)_v10)[9];
}

cc: @AndyAyersMS, @jcouv

[gfoidl]

Small nit in spacing of the comments