Run a YAML spec over the source

[peterbjohnson]

Layer 1 of a draft. A spec is a small YAML file of selectors over the pandoc AST that says which blocks are questions, parts and solutions, with strip patterns, ignore rules and a layout. The four filters stay exactly as they are and are what the layout field names: a spec with layout: PartsSepSol applies that filter's logic to the blocks the selectors picked out. The worked example in the agent spec shows the shape:

question: Header level=2 text~"^Question"
part:     Para text~"^\([a-z]\)"
solution: after Header text="Solutions", label~"^\d+(\([a-z]\))?"
strip:    ["^\([a-z]\) ", "^\d+(\([a-z]\))? "]
ignore:   Header level=1
layout:   PartsSepSol

in2lambda spec run SPEC fills the draft's fields with provenance, records the spec's hash in the draft, and returns coverage: which blocks are in no field and not ignored. Done when each saved spec plus its source in the test data produces its expected draft, so a new pair dropped in the test data is a new test with no code, and a spec that names an unknown layout or selector is refused with the line.

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Workbench ticket t26.