More flexible result matching

[nathanalderson]

This PR adds support for more flexible matching on the return value and raised exception for determining whether to retry. Instead of only supporting a list of atoms for each, you can now provide any of the following:

• An atom (for example: :not_okay, SomeStruct, or CustomError). In this case, the do block will be retried in any of the following cases:
  • The atom itself is returned
  • The atom is returned in the first position of a two-tuple (for example, {:not_okay, _})
  • A struct of that type is returned/raised
• The special atom :all. In this case, all values/exceptions will be retried.
• A function (for example: fn val -> val.starts_with("ok") end) or partial function (for example: fn {:error, %SomeStruct{reason: "busy"}} -> true). The function will be called with the return value and the do block will be retried if the function returns a truthy value. If the function returns a falsy value or if no function clause matches, the do block will not be retried.
• A list of any of the above. The do block will be retried if any of the items in the list matches.

This should address both #54 and #56.

[nathanalderson]

How are we feeling about this? Anything I can do to push it forward?

[nathanalderson]

Periodic bump. Any feedback? Can we merge?

[greenfork]

Works great, I'm using your branch @nathanalderson , thank you for this patch!

[nathanalderson]

Thanks for merging!