[V1] Consistent naming on AST literal, DataType, PType, and Datum methods.

[rchowell]

Description

We have many APIs for the various values and types which are almost consistent and we've covered a lot of this in the latest round of cleanup PRs. There a few remaining items like

• .null() not possible so .nil(), .nul() vs. `.nullValue()

Which vary across the codebase; this would be nice to standardize since we can't/won't change post Jan.

DoD (Definition of Done)

List set of criteria that characterize this issue as DONE or COMPLETED.