* 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
Adam Perlin