Consider asymmetric key import/export API for NS2.1

[bartonjs]

https://github.com/dotnet/corefx/issues/20414 significantly helps .NET applications work with asymmetric keys "the Linux way", as well as other generally useful in hybrid environments scenarios.

Types to diff against corefx/3.0:

• System.Security.Cryptography.AsymmetricAlgorithm
• System.Security.Cryptography.RSA
• System.Security.Cryptography.ECDsa
• System.Security.Cryptography.ECDiffieHellman

(and potentially the deriving types, for their overrides)