* Add Flux Rename, Drop, and Keep functions.
* Add flux language tests for drop and rename; bugfixes around constructing drop/rename procedures
* Correct autcomplete error, remove debug statements
* Fix additional autocomplete error
* Add support for fn parameter to flux drop and rename functions
* Remove unneeded renameDropTransformation field
* WIP initial thoughts about merge/procedure rewrite logic
* Remove unnecessary external columnFn types
* Add additional query tests for drop/rename, add error when drop or rename refers to column that doesn't exist
* Change swapped test names
* Add keep function implementation
* Refactor rename/drop implementation to make use of helper methods
* Remove test skips forqueries that produce an expected error
* Make predicate return value logic clearer in drop implementation
* Add group key support to drop/rename/keep, update unit and query tests accordingly
* WIP refactor drop and rename to use new SchemaMutator interface
* Add unit tests for refactored drop, keep, and rename; bug fixes
* Update file structure for flux schema mutation functions
* Refactor mutation functions and interface
* Address final review drop/rename PR review comments
* Add builtin import to make sure builtins are initialized if test drop_rename_keep_test is run as standalone
* Small aesthetic change to schema_mutators.go
These tests were part of a PR when the rename was made.
The changes were not rebased before merge so we did not discover the
failures till after the merge.
previous versions only supported the first parameter to time() that set the window size. This version supports the second parameter, which shifts the offset a fixed amount from the epoch