Replace DeterministicECDSA by ECDsa

[vermorel]

The implementation DeterministicECDSA should be replaced by its BCL counterpart ECDsa.

[osman-turan]

This issue requires to replace BigInteger of BouncyCastle by BCL counterpart as well. So, effectively this issue requires almost removing entire BouncyCastle from the codebase as a bonus which is a definitely nice thing. Although I made some progress, I hit a point where I needed ASN.1 DER serialization and deserialization. I wanted to use BCL classes for this purpose, but there isn't any mechanism so far. When I was looking .NET Core repository, I encountered following ticket: https://github.com/dotnet/corefx/issues/21833

In short, this ticket points AsnReader and AsnWriter classes which seems well-optimized (e.g. uses ReadOnlyMemory<T> class) should be made public (internal for now). API has not been finalized yet. But, I believe they will make it eventually. So, far we have following options:

1. Implement a very-narrow implementation (only use required parts) of DER encoding/decoding based on BouncyCastle.
2. Copy & paste AsnReader and AsnWriter classes from .NET Core repository and use until API is finalized.
3. Implement a helper class to invoke internal AsnReader and AsnWriter classes with reflection (I dislike this idea for several reasons, but it's still an option).
4. Use System.Security.Cryptography.Asn1.Experimental NuGet package until API is finalized ( https://dotnet.myget.org/feed/dotnet-corefxlab/package/nuget/System.Security.Cryptography.Asn1.Experimental ).

What do you think?

[vermorel]

I suggest option (2) copy-and-paste, and keep a comment as well as an URL pointing out to the .NET Core counterpart and hinting for the replacement to be done in the future.

[osman-turan]

This issue is more trouble than it seems due to deep integration of BouncyCastle types (I simply gave up after some point). I propose to postpone this issue for a while until all unnecessary parts have been excluded.