fm_renumber performance

`fm_renumber()` in `fieldmanager.js` performs a lot of DOM traversal, and it would appear that much of it is duplicative in deeply nested groups and should be unnecessary.

This function should be refactored to, ideally, only visit each field once. Second best option is that each field is visited only the number of times as it has incrementable keys (as denoted by `data-fm-array-position` in `.fm-wrapper` nodes). As it presently stands, each field is visited an exponential number of times based on its depth and the ancestral group structure.

Also see #827, #828