[Gandiva] [Python] Make it possible to list all functions registered with Gandiva

[asfimport]

This will also be useful for documentation purposes (right now it is not very easy to get a list of all the functions that are registered).

Reporter: Philipp Moritz / @pcmoritz
Assignee: Philipp Moritz / @pcmoritz

PRs and other links:

• GitHub Pull Request #2933

_{Note: This issue was originally created as ARROW-3746. Please see the migration documentation for further details.}

[asfimport]

Philipp Moritz / @pcmoritz:
Issue resolved by pull request 2933
#2933