First version of PolyKit published!

[rdeago]

It took a little (in geological terms 😁) longer than expected, but here it is: yet another polyfill for C#, but done both ways so you don't have to choose between a library and a source package.

I suck at marketing hate repeating myself, so here's the README.

Tagging @kzu as agreed in devlooped/ThisAssembly#105 (comment)