Fix the constructor of Snuffle base class

[moien007]

The Snuffle base class's constructor's first parameter is in byte[] key which doesn't make sense, in is same as ref keyword (which means a reference to the specified value rather than value itself as you know) with the difference that in doesn't allow modifications to the value that being referenced (at the language level).

https://github.com/idaviddesmet/NaCl.Core/blob/54372a2914d9f770d1bd3c42898fb17928f57a28/src/NaCl.Core/Base/Snuffle.cs#L40

So what is happening here in this constructor is that we are passing a reference to a byte array (which arrays are references too) so the goal we are looking for (a read-only input or const char *key in C) is not achieved here. In order to fix this problem, we have to replace the in byte[] key with ReadOnlyMemory<byte> key which introduces a lot of changes in the code plus it is a breaking change.

Regards.

[daviddesmet]

Good spot! This was somehow slip away when implementing Span. Will fix it in the next commit.