Upgrade to .NET Core 3.1

.NET Core 3.1 is the LTS release, so update to that for the time being. Also, now up to date on the MSTest NuGet package.

Fix GreaterThan and LessThan.

Previously, GreaterThan and LessThan had some special behavior where despite looking like they took a top argument, they would only look at the top argument and treat it like a bottom argument. This is now resolved, and the only example that used these, prime.fl, has been updated to match.

First release!

Fluency is finally ready for its first release! Download yourself some fluency.