We should have a function for coercing an object to another type and class, in a efficient way, and class aware (for double type, different classes uses different NA representation so we need to handle that).
It is important because coercion is a very common operation, especially num-int or int-num for user convenience so they don't have to type L suffix, but there are tons of other applications.

Initial work on that can be found in #3765 (generic coerceClass)

No-alloc coerce written for C assign function #3909 is good foundation, that potentially could be extracted out to an standalone function.