Avoid some virtual dispatch in Encodings

[stephentoub]

• Seal some internal overrides
• In the more common encodings, add overrides of GetBytes/GetByteCount/GetChars/GetCharCount to avoid another layer of virtuals from the base class (they can call to an internal non-virtual).

Related to https://github.com/dotnet/corefx/issues/30382

cc: @jkotas, @roji, @tarekgh, @krwq

[stephentoub]

Example microbenchmark:

using System;
using System.Diagnostics;
using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using BenchmarkDotNet.Attributes.Jobs;

[MemoryDiagnoser]
[InProcess]
public class Benchmarks
{
    private static void Main() => BenchmarkRunner.Run<Benchmarks>();

    private char[] _chars = "hello world".ToCharArray();
    private byte[] _bytes = new byte[100];

    [Benchmark]
    public void EncodingUTF8GetBytes()
    {
        ReadOnlySpan<char> src = _chars;
        Span<byte> dst = _bytes;
        for (int i = 0; i < 1000; i++)
        {
            Encoding.UTF8.GetBytes(src, dst);
        }
    }
}

Method	Mean	Error	StdDev	Allocated
Before	21.43 us	0.1606 us	0.1502 us	0 B
After	19.53 us	0.1589 us	0.1409 us	0 B