Mango with promises

[aferreira]

This pull request adds promise-returning methods to Mango and related classes as an alternative to non-blocking with callbacks. For this, it relies on Mojo::Promise as introduced with Mojolicious 7.53.

All of Mango continues to work with older Mojolicious as it used to. Only, if Mojo::Promise is not available, the new methods, like $collection->find_one_p and $bulk->execute_p, don't do anything but to throw exceptions like:

find_one_p() requires Mojo::Promise

In the same vein, tests cover the new methods if Mojo::Promise is available and, if not, the corresponding tests are ignored.

Most of the patch is repetitive documentation and the new tests. The code change is small, consisting of a new (internal) helper class Mango::Promises that generate the _p methods from the original methods (like Mango::Bulk::execute_p from Mango::Bulk::execute) and the inclusion of code like below to each class

package Mango::Collection;
...
use Mango::Promises;
...
Mango::Promises->generate_p_methods(
  qw(aggregate create/0 drop/0 drop_index/0 ensure_index/0 find_and_modify find_one
    index_information insert map_reduce options remove rename save stats update)
);

[aferreira]

@oliwer Let me know what you think about this patch.

[oliwer]

From a quick first look this looks very good! I'd like to test it a bit more before merging. I'll comment back soon.