Add ECDsa span-based methods

[stephentoub]

cc: @bartonjs
Closes https://github.com/dotnet/corefx/issues/22615

[bartonjs]

What here (and at VerifyHash) requires the unsafe keyword?

[stephentoub]

The null argument to TrySignHash is a void*.

[stephentoub]

Thanks for reviewing, @bartonjs.

[ektrah]

@stephentoub - Quick question: The code currently pins spans for P/Invoke, e.g.:

private static unsafe int RsaEncryptPkcs(
    SafeSecKeyRefHandle publicKey,
    ReadOnlySpan<byte> pbData,
    int cbData,
    out SafeCFDataHandle pEncryptedOut,
    out SafeCFErrorHandle pErrorOut)
{
    fixed (byte* pbDataPtr = &pbData.DangerousGetPinnableReference())
    {
        return RsaEncryptPkcs(publicKey, pbDataPtr, cbData, out pEncryptedOut, out pErrorOut);
    }
}

Is that required or could the P/Invoke method defined to accept a ref byte? E.g.:

private static int RsaEncryptPkcs(
    SafeSecKeyRefHandle publicKey,
    ReadOnlySpan<byte> pbData,
    out SafeCFDataHandle pEncryptedOut,
    out SafeCFErrorHandle pErrorOut)
{
    return RsaEncryptPkcs(publicKey, ref pbData.DangerousGetPinnableReference(), pbData.Length, out pEncryptedOut, out pErrorOut);
}

[DllImport(Libraries.AppleCryptoNative, EntryPoint = "AppleCryptoNative_RsaEncryptPkcs")]
private static extern int RsaEncryptPkcs(
    SafeSecKeyRefHandle publicKey,
    ref byte pbData,
    int cbData,
    out SafeCFDataHandle pEncryptedOut,
    out SafeCFErrorHandle pErrorOut);

I'm wondering because manual pinning is usually not required when passing, for example, a field by reference:

class A { public int B; }

[DllImport(...)]
static extern void Foo(ref int value);

static void Main()
{
    var a = new A();
    Foo(ref a.B);
}

[stephentoub]

Is that required or could the P/Invoke method defined to accept a ref byte?

That's a good question. I actually hadn't considered doing that, but I don't see why it wouldn't work. I'll try and submit a PR to clean it up assuming it works.