Add Enum schema strategy

[mbercx]

Fixes #46

Add a new subclass of SchemaStrategy for supporting the 'enum`
keyword.

The Enum class keeps track of a set that contains all of the values
that were provided by the 'enum' keywords in the schemas. When the
builder is converted to a schema, this set is converted to a list.

[mbercx]

@wolverdude I just set match_object to return False for now since I'm not sure how to implement this. Do we restrict the possible values to only those obtained from the object added? You mentioned using a seed schema in #46, but can I check if the existing schema already has an 'enum' defined?

[wolverdude]

Hey, thanks for doing this! I'm not sure a separate Strategy class is warranted here though. Strategies are generally based on type, but enum is a generic keyword, so it could apply to any type. Perhaps a better strategy would be to apply this logic to the base strategy, but we also have to deal with the edge-case where there are enums from several different strategies that need to be combined, so this would require some updates to the Node class as well.

Given the complexity, it may be best if I implement this myself, but I do think support for enum is a useful feature that fits the constraints and purpose of the library.

Unrelatedly, the test failures are not your fault, but I just merged a fix for them, so just merge in master, and they should go green. Please add tests that cover all use cases related to any new features you add.

[wolverdude]

This PR has been inactive for months. @mbercx do you intend to merge this or should I close it?

[mbercx]

Hi @wolverdude, sorry for the slow response! I won't have time to adapt the PR to your suggestion of adding the logic to the base class, especially since it's been a while since I looked at this. 😅 So feel free to close the PR if you think that is the better approach!

[wolverdude]

Thanks. I'll wait for upvotes or implement Enum if/when I get around to it.