Underlying type fix

[mfleader]

Changes introduced with this PR

'underlying type' == subtype

• Fix (un)serialization of map data
• Add discriminator_inlined field to OneOfSchema to allow users to declare the OneOfSchema's discriminator as a field in their subtype, and give it a default setting of false
• Add function to validate whether or not the OneOfSchema's subtypes have been written correctly
• Fix nil interfaceType bug where interfaceType is not set when a OneOfSchema is created without using a OneOfSchema constructor (NewOneOfIntSchema[T](), NewOneOfStringSchema[T]()).
• Add unit tests to cover new behavior
• Increase test coverage of error conditions

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[jaredoconnell]

Looks good once all of Webb's conversations are resolved. Remember to make sure the release is a pre-release. This is a potentially breaking change.

[webbnh]

I think this resolves all my comments (I'm not sure that you actually addressed my testing concerns, but the code that they referred to is all gone (I think), so I guess it's all good...).

I have two small comments which you can ignore if you like.

[webbnh]

This is good to go, unless there's a change to address my previous.