Make pub(crate) fields of `ASTWithValidityInfo` private.

[ratmice]

This makes some fields which were pub (crate) private, and replaces their usage outside the ast module with the existing pub functions. Which makes it a tiny bit easier to refactor these fields.

[ratmice]

Also noticed in that last commit (even though the private fields are visible at that point), that can be implemented in terms of the pub api.

Probably worth squashing.

[ltratt]

Please squash.

[ratmice]

Squashed.