Fixes for v0.7

[fredrikekre]

Most notably removes dependency on Nullables and Compat.

[stevengj]

I would have preferred to keep 0.6 support until a little while after 0.7 is released, and then have a separate PR dropping compat...

[fredrikekre]

Fair enough, should we then wait with the Nullable{T} -> Union{T, Nothing} change too?

[KristofferC]

Last functional commit here was four months ago and it is always possible to backport something and release from another branch if the need arises.

[Keno]

I agree, this package is fairly stable and if we're doing a somewhat breaking change in this package (Nullable -> Union{T, Nothing}), it would be good to get that in early, so other packages can adjust to these changes. Otherwise we'll have a second wave of instability as we upgrade packages. Releasing a hotfix from a branch should be just fine.

[Keno]

@stevengj are you ok with that approach? Lots of packages depend on JSON.jl, so it would be good to get a compatible version out soon.

[stevengj]

Okay.

[Keno]

Thanks @fredrikekre. I'll kick off another PkgEval run as soon as this is merged/tagged.