Exported Fields in Packages

[DavidVorick]

Unfortunately, this is a rather small but many-line issue.

Packages other than siacore should not be able to modify anything within the State, Blocks, or BlockNodes, but need to be able to query all of it... which means unexporting all of the fields and writing a ton of getters.

Kind of gross but ultimately an important move.

We'll hold off for the time being.