Deeply merges two maplike structures into one.

All keys in b will be added to a, overriding any existing one (i.e., the keys in b “have precedence” over the ones in a).

Any map or list is considered a child container, and when found, the algorithm willl descend into these structures.

As a matter of convenience, the second argument may be nil, which will cause the function to return the first argument unmodified.