This ns is mostly just for documentation and for providing an alias.

Defines standard keyword keys used in the implementations ('impls') map of a
TransientPolyMap to override default transient map behaviors.

Each var defined here holds the actual namespaced keyword as its value
(e.g., `assoc_k_v` evaluates to `::assoc_k_v` in this ns).
The documentation for the override is attached as metadata to the var.

Example expected signature for `::assoc_k_v`:
`(fn [this t_m impls metadata k v] ...)`

Overrides for mutating operations (`assoc!`, `conj!`, `dissoc!`)
should generally return the `this` (the TransientPolyMap instance) to allow chaining.
The override for `persistent!` should return the new persistent `PolyMap` with all
internal maps persisted as well.
Overrides for read operations should return the requested value.