thema: Flatten schemas to one-dimensional array

[sdboyer]

After a suggestion from @myitcv at Kubecon last week, the light finally clicked on with how we can get rid of the nested array in seqs, add reverse lenses to the structure, and overall just drastically improve UX. In fact, we can get rid of the concept of "sequence" entirely.

As an example, the final form for the narrowing exemplar is this:

schemas: [{
	version: [0, 0]
	schema: boolish: "true" | "false" | bool | string
}, {
	version: [1, 0]
	schema: properbool: bool
}]

lenses: [{
	to: [0, 0]
	from: [1, 0]
	input: _
	result: {
		// Preserving precise original form is a non-goal of thema in general.
		boolish: input.properbool
	}
	lacunas: []
}, {
	to: [1, 0]
	from: [0, 0]
	input: _
	result: {
		if ((input.boolish & string) != _|_) {
			properbool: input.boolish == "true"
		}
		if ((input.boolish & bool) != _|_) {
			properbool: input.boolish
		}
	}
	lacunas: [
		{
			sourceFields: [{
				path:  "boolish"
				value: input.boolish
			}]
			targetFields: [{
				path:  "properbool"
				value: result.properbool
			}]
			message: "boolish was a string but neither \"true\" nor \"false\"; fallback to treating as false"
			type:    thema.#LacunaTypes.LossyFieldMapping
			cond:    ((input.boolish & string) != _|_) && ((input.boolish & ("true" | "false")) == _|_)
		},
	]
}]

seqs is gone, nested arrays are gone. version is an explicit field within the #SchemaDef (though there's still no actual choice to be made by the user - it's all governed by backwards compat rules) Additional fields are possible in there, including providing examples of the schema.

Lenses are moved into their own, separate structure, independent of the schemas list. They're a bit more verbose than i'd like, with it still be necessary to explicitly write the input field. But from and to are now just a version number, rather than needing to be a reference to the correct schema. Better names for the other fields, too - i think result is indicative of what's supposed to go there, though i don't think this has reached the level of self-explanatory quite yet.

Fixes #2
Fixes #18

• Add native CUE check that lens set is complete
• Add more native CUE test cases
• Convert existing exemplars, tests
• Refactor Go packages to work with new CUE structures
• Introduce AST-based migrator

[sdboyer]

This is finally, finally good to go. Lots of follow-ups, of course.