Fix Secp256k1 on 64bit Linux

The bindings to the native library functions were incorrectly using C#
uint in place of C's size_t. This was causing the library to segfault on
64bit linux due to C#'s uint being 32bit where C's size_t is 64bit.

This commit changes these bindings to use UIntPtr as a closer
approximation of size_t.

Author: canndrew

src/Secp256k1.Net/Interop.cs

@@ -357,4 +357,4 @@ public enum Flags : uint
     }
 
 
-}
+}