[Python] Implement Table.cast for casting from one schema to another (if possible)

[asfimport]

This is an API convenience that would be very useful. See example use case in ARROW-2926

Reporter: Wes McKinney / @wesm
Assignee: Krisztian Szucs / @kszucs

PRs and other links:

• GitHub Pull Request #2530

_{Note: This issue was originally created as ARROW-2936. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Issue resolved by pull request 2530
#2530