codegen: Support default values in schema objects

[hughsimpson]

Adds support for default values in schema objects. Actually just adds the default value to the constructed case class and expects the json lib to handle the rest of the functionality. Optional fields with no explicit default in the schema are treated as having a default of None. Constructing the representation of default values traverses the schema tree, and so should generate valid syntax for all currently supported spec objects.

[kciesielski]

Thanks! What would happen for nested fields with default values? Would they just be ignored, or should we expect some errors?

[hughsimpson]

In general nested fields would only work if the actual type was the guessed type from parsing the default value. I think the render def would need to take the full schema model to be able to resolve that limitation... As it stands, in cases where we guessed wrong, then the generated code should fail to compile

[kciesielski]

Thank for clarifying. I'm ok with going forward with this solution. In case we have users with really sophisticated schemas not covered by this, the implementation could be revisited for enhancements, but IMO the current scope is good enough :)
Happy to merge after the conflicts are resolved.

[hughsimpson]

Thanks! I'm currently working with a fairly large spec and haven't run into any examples where the defaults this generates breaks compilation, but I'll revisit this if anyone raises an issue once it's released (and may do so anyway, depending on how the wind blows)

Edit: I took a look at making it more principled, and actually it wasn't too nightmarish in the end. It should now handle complex default values, and the choice of repr for 'stringy' values is now considerably less arbitrary. I think it looks a lot nicer now.

[kciesielski]

Excellent, thank you for this contribution @hughsimpson!