Dinstinguish between within-crate export, and crate-external export

[eholk]

Sometimes we'll have modules within a library/crate that needs to refer to "private" things from other modules, but we may not want to export as part of the public API. We should have a way to support this.

[brson]

This is discussed in #1893. There's probably going to be a crate visibility modifier.

[catamorphism]

Since it appears this is part of #1893, closing this one.