Project files improvement and more target-frameworks

[moien007]

Hi,

This PR contains the following changes:

• Improvement and simplification of projects files
• Updating NuGet package references to the latest available
• Support for .NET Framework 4.8
• Support for .NET Standard 2.1
• Using BitOperations class for .NET Core 3.1 (The explanation)

These changes are not what I supposed to make but I have to commit that I may not be able to solve the two issues #40 and #41 at all due to my lack of knowledge of the NaCl itself, sorry for that.

Regards.

[daviddesmet]

Thanks for your PR. I've added some comments in the code before merging your changes.
Could you please take a look into them? For #40 and #41, I will take a look later. I still have pending a push for #32

[moien007]

I'm a noob in git. I addressed your requested changes by pushing this commit, is that how I should have done that?

[daviddesmet]

I'm a noob in git. I addressed your requested changes by pushing this commit, is that how I should have done that?

No worries, you did it right! We all started as noob in git. 😉
Will merge your PR and address the CI Pipeline later since it's giving some errors.

[daviddesmet]

@moien007 The changes will be pushed later to nuget.org but in case you feel eager to try it now, you can grab them from here.