Add AssymetricAlgorithm Span-based APIs

[stephentoub]

Separated out of https://github.com/dotnet/corefx/issues/21281 for tracking purposes.

• Implement/expose/test RSA methods
• Implement/expose/test DSA methods
• Implement/expose/test ECDsa methods

namespace System.Security.Cryptography
{
    public abstract class RSA : AsymmetricAlgorithm
    {
        public virtual bool TryDecrypt(ReadOnlySpan<byte> source, Span<byte> destination, RSAEncryptionPadding padding, out int bytesWritten);
        public virtual bool TryEncrypt(ReadOnlySpan<byte> source, Span<byte> destination, RSAEncryptionPadding padding, out int bytesWritten);
        protected virtual bool TryHashData(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, out int bytesWritten);
        public virtual bool TrySignData(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, RSASignaturePadding padding, out int bytesWritten);
        public virtual bool TrySignHash(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, RSASignaturePadding padding, out int bytesWritten);
        public virtual bool VerifyData(ReadOnlySpan<byte> data, ReadOnlySpan<byte> signature, HashAlgorithmName hashAlgorithm, RSASignaturePadding padding);
        public virtual bool VerifyHash(ReadOnlySpan<byte> hash, ReadOnlySpan<byte> signature, HashAlgorithmName hashAlgorithm, RSASignaturePadding padding);
        …
    }

    public abstract class DSA : AsymmetricAlgorithm
    {
        public virtual bool TryCreateSignature(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesWritten);
        protected virtual bool TryHashData(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, out int bytesWritten);
        public virtual bool TrySignData(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, out int bytesWritten);
        public virtual bool VerifyData(ReadOnlySpan<byte> data, ReadOnlySpan<byte> signature, HashAlgorithmName hashAlgorithm);
        public virtual bool VerifySignature(ReadOnlySpan<byte> rgbHash, ReadOnlySpan<byte> rgbSignature);
        …
    }

    public abstract class ECDsa : AsymmetricAlgorithm
    {
        protected virtual bool TryHashData(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, out int bytesWritten);
        public virtual bool TrySignData(ReadOnlySpan<byte> source, Span<byte> destination, HashAlgorithmName hashAlgorithm, out int bytesWritten);
        public virtual bool TrySignHash(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesWritten);
        public virtual bool VerifyData(ReadOnlySpan<byte> data, ReadOnlySpan<byte> signature, HashAlgorithmName hashAlgorithm);
        public virtual bool VerifyHash(ReadOnlySpan<byte> hash, ReadOnlySpan<byte> signature);
        …
    }
}

This includes not only adding the virtuals but also providing meaningful and efficient implementations on the internal concrete implementations returned from the various Create methods.