use generic pointers

[Stebalien]

~It sucks, but, as far as I know, this is the only way to avoid reflection and other pointy nastiness.~

After applying @Wondertan, it's less terrible.

[Stebalien]

See https://medium.com/@reetas/using-generics-with-pointer-receivers-in-go-39ee237d7475

[rvagg]

I've pulled this in to rvagg/generics and made some additional modifications, including creating and holding the zero value on the Node and its children instead of making a new one every time we pass through modifyValue(). (Although, I can't say I measured a huge perf difference when doing this in https://github.com/filecoin-project/go-state-types/pull/298).