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ARROW-17349: [C++] Allow casting map types (#14198)

* Implements casting functions from `MapType` -> `[MapType, list<struct>, large_list<struct>]`, with support for mapping the field names to new ones.
* Adds a new `Datum::View()` method to map an array to a new type.
* Use `Datum::View()` in early return of cast dispatching (if types are "equal"). Right now this is useful for mapping field names for map arrays, but it could also be used for list types once we enable some way to check if list types are equal except for its field name. See also: https://issues.apache.org/jira/browse/ARROW-14999

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>