resolve: make signature reflect impurity

[cstrahan]

resolve will return different results depending on the current
working directory, so this should be captured in the type signature.

Also, simplify the arguments: zero arguments is the same as giving "."
as the only argument, so from and to can be collapsed into one
argument.

[hdgarrood]

👍 I will merge this manually.

[hdgarrood]

Actually, on second thoughts, I'm not convinced about collapsing from and to into one argument. In particular, it departs from how the node api is documented, so the node api docs are no longer as useful for understanding this. I also think it's a probably good thing that you have to pass at least one FilePath in; Path.resolve [] giving you the current directory seems a bit strange to me, and it's probably better to use Process.cwd for that.