Map functions

Build maps and read their keys, values and entries.

Function	Type	What it does
`map`	Scalar	Constructs a map from alternating keys and values, or from parallel arrays of keys and values.
`map_coalesce_extract`	Scalar	Returns the first non-NULL matching map value according to map and key priority.
`map_entries`	Scalar	Returns map entries as an array of key/value structs.
`map_from_arrays`	Scalar	Builds a map by pairing corresponding keys and values.
`map_from_entries`	Scalar	Builds a map from an array of two-field structs.
`map_keys`	Scalar	Returns the keys of a map as an array.
`map_values`	Scalar	Returns the values of a map as an array.