Constrain sequence lenses so that an absent declaration is not valid

[sdboyer]

A key invariant of scuemata is that authors must define lenses for all sequences after the first. While it's impossible to generically verify the completeness of the translations in these lenses, or the lacunae the may emit, the bare minimum here is erroring if the author doesn't define a lens, at all.

Unfortunately, when we take the #Lens definition

thema/lineage.cue

Lines 48 to 67 in b7b4330

	#Lens: {
	// The last schema in the previous sequence; logical predecessor
	ancestor: joinSchema
	// The first schema in this sequence; logical successor
	descendant: joinSchema
	forward: {
	to: descendant
	from: ancestor
	rel: descendant
	lacunas: [...#Lacuna]
	translated: to & rel
	}
	reverse: {
	to: ancestor
	from: descendant
	rel: ancestor
	lacunas: [...#Lacuna]
	translated: to & rel
	}
	}

and reference it directly in seqs where we want lens instances to appear:

thema/lineage.cue

Lines 72 to 80 in b7b4330

	seqs: [
	{
	schemas: #Sequence
	},
	...{
	schemas: #Sequence
	lens: #Lens
	}
	]

it means that a lineage is still valid even when the author omits the field. The questions are:

• What's a reasonable thing to force a constraint failure on? Should #Lens itself be refactored so that it's more amenable to doing this?
• How do we square wanting this to be incomplete with also wanting to provide helpers that reduce some of the boilerplate of writing lenses?

Maybe this is just yet another place where we need a must() check (cue-lang/cue#943), particularly to emit the right error.

Note that this issue is targeting a relatively minimal check - to climb up to our true-goal invariant, in addition to basic lens existence, we have to be certain that a) the Lens connects the end of the prior sequence to the head of its current sequence, and b) that given a concrete input valid wrt the predecessor schema, the rel of both forward and reverse lenses is sufficient to produce a concrete output of the successor schema.

[sdboyer]

#82 will address this problem; we won't address it in current structure, which is pretty broken wrt lenses anyway.