Fix NewValue checks to be properly recursive

[paddycarver]

Fix the checks that NewValue runs to ensure that a value can be used
with a Type to properly handle DynamicPseudoTypes nested in complex
types. Builds on and supersedes #76.

Should we make this part of the behavior for .Is? My gut says "no",
because Is is supposed to be about checking whether a type is
semantically similar, not that it can be used as another type. Maybe we
want a Fulfills method, similar to Is, that does this? I don't know
that this behavior needs to be exported, though...

Fixes a bug in #74 that would panic for complex types containing
DynamicPseudoTypes.

[kmoe]

Looks good. We can always export UseTypeAs/Fulfils or change Is behaviour later.

[paddycarver]

I would like to, in 0.4.0, do some work on Is to make this cleaner and have less magic behavior:

• Is is a shallow check, that root types are the same type
• Equals is a deep check, that root types and all elements and attributes are the same type
• ConformsTo is a deep check, that root types and all elements and attributes conform (are equivalent, or a concrete type is being substituted for DynamicPseudoType)

I just don't want to hold up 0.3.0 for that work, it's a big enough release already. I'll write up an issue for it.

[github-actions]

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