Inserts `v` into `ks` after `before-ks` in `omap`.
`omap` is a nested associative structure, especially a map that can keep the order: `array-map`, [ordered-map](https://github.com/clj-commons/ordered), etc.
`array-map` cannot keep the order when there are many elements, so in that case, please consider using `ordered-map` etc.
`v` is new value, `ks` and `before-ks` are sequences of keys.