[FEA] Support Map as input to explode and pos_explode

[revans2]

Is your feature request related to a problem? Please describe.
This comes from an audit of missing functionality for nested types. Spark supports explode and pos_explode on maps. We should be able to support them too as we store a map as an array of key/value structs so we should be able to do the explode on the array in the map, and then pull out the key/value columns from the resulting struct. This should require no changes to cudf to support this.