Add function to associate keyvals in objects

Defines `lt.object/associate!`, a new function that calls `assoc-in` on the data structure of an LT object. (Nothing too important; just a small addition for caller convenience.)

Functionally, it is a subset of `object/merge!`, but it allows to update or create keyvals in a more readable fashion when the pre-existing value is irrelevant. E.g.:
```clojure
(object/associate! obj [:deeply :nested :key :value] 1)
```
rather than
```clojure
(object/merge! obj {:deeply {:nested {:key {:value 1}}}})
```