Keep static constructors

[MichalStrehovsky]

We should ensure static constructors are kept, ideally mirroring the rules for when the static constructor is triggered. E.g. if the type is beforefieldinit, we only need to keep the static constructor if any of the static fields are kept.

The type system might come handy because there are APIs to get/check whether the static constructor is there and get the actual method.

[MichalStrehovsky]

We should undo what #92 does and replace with the better thing