Add getOrPut equivilant to PersistentMap

[rnett]

getOrPut is a commonly used function on mutable maps. I would like to have a function to fill the same niche on PersistentMap - creating and adding the new value only if the existing value is not present. This can be done using a contains check and then a put, but that requires two scans of the map, while this method should only require one.

In particular, I'd like it if the method:

• inlined the lambda
• Returned both the new map, and the updated value for the key

[DmitryNekrasov]

Before we settle on a signature, could you share a concrete call site where you'd reach for this? Seeing what you do with both the value and the resulting map afterwards would help, since that decides whether returning a pair is the right shape here.

[rnett]

It's been long enough I don't have the exact code handy, but I'm pretty sure it was storing the persistent map in a var or atomic variable and using it like you would a mutable map.