[BUG] Remaining testing and cleanup tasks for Abstract Syntax Tree

[harrism]

PR #5494 left a few loose ends. This issue tracks them.

• AST / n-ary transform need tests with larger data sizes. Often issues arise in logic with custom kernels pertaining to differences between running a kernel with a single block and with multiple blocks. Testing with enough data to cause multiple blocks to run is necessary for this reason.
• ast_plan needs docs
• Move ast_plan initialization logic from compuite_column (in transform.cu) into ast_plan constructor.
• expression constructors need SFINAE changes for correct functionality.

CC @bdice

[lamarrr]

I attempted a fix for the fourth one, here: #5494 (comment)

[github-actions]

This issue has been marked rotten due to no recent activity in the past 90d. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.

[harrism]

@codereport assigning this to you since you are looking at AST. Please let @jrhemstad and I know if that's not appropriate.