Expanded Bitcoin Variable Length classes.  Built out VarInt class.  Refactored VarStrings to use VarInt.  Split out extension methods for converting to VarInt & VarStrings.  Updated Demo program to make use of new classes.
Moved and renamed BigIntExtensions to be consistent
Updated to use VarString extension methods
Reafactoring on EcPoint (DRY).  Commented out unused local variables.  To be remove in future commits.
Removed backup hard coded strings in Secp256k1.  Added hex string unit tests.  Included updated csproj excluded from prior commits.