add basic CI using github actions

[Vindaar]

Let's see if this works.

Locally the tests are broken for me, which is why I wanted to set up CI.
edit: locally works fine now (nimpy setup issue), but C++ backend also causes codegen errors.

edit:
Updated to be more modern. Should work.

[Vindaar]

Merging despite the CI failure. Better set up a CI that fails than not having any...

The issue relates to the Nim compiler getting very confused in generics once again. Arraymancer nowadays has a toTensor that takes SomeSet[T]. The combination of regular generic T with a type class SomeSet = HashSet | OrderedSet makes it try to compile code as a SomeSet where it really does not make any sense. This needs to be fixed in arraymancer by splitting the SomeSet explicitly.