* First pass at modular wiki indexes
An exploratory experiment
* Fix tests
* Faster checking for existence of index methods
We don't really need to check the type
* Use the index for the has operator
* Fix typo
* Move iterator index methods into indexer modules
Now boot.js doesn't know the core indexers
* Fix up the other iterator index functions
* Fix crash with missing index branch
* Limit the field indexer to values less than 128 characters
* Fallback to the old manual scan if the index method returns null
* Sadly, we can no longe re-use the field indexer to accelerate the `has` operator, because the index now omits tiddlers that have field values longer than the limit
Still need to make the index configuration exposed somehow
* Rearrange tests so that we can test with and without indexers
We also need to expose the list of enabled indexers as a config option
* Test the field indexer with different length fields
So that we test the indexed and non-indexed codepaths
* Added better handling for sortByList manual placements
If manual placement specifications show up in an inconvenient order,
sortByList, will go to the trouble of processing them in that order.
* Added tests to confirm solution to (#3296)
...That custom tag ordering will not choke when tiddlers get sorted after their dependencies have been placed around them
* Corrected list-after bug when referencing external titles
* Using more error-proof $tw.utils.hop in sortByList
* minor indentation correction in test-tags.js
