S7: report commands — check, explain, optimize, trace, bundle, ir, provenance

Part of #17. Design: docs/design.md §3, §4.

Scope

Each view of a script is a command that writes one thing to stdout.

Command	stdout	API it uses
`check`	nothing; diagnostics on stderr; `--json` for one document on stdout. Exit 0 or 3.	`validate`
`explain`	the physical plan (`--json`)	`explain`, `explainJson`
`optimize`	the rewritten Relix text, so it works as a filter (`relix optimize q.relix > fast.relix`); `--report` for the rules fired and the before/after trees	`optimized().render()`, `rewrites()`, `OptimizationReport`
`trace`	the event feed (rows discarded)	`stream(listener)`
`bundle`	the playground JSON bundle, still produced for a broken script	`QueryBundleJson`
`ir`	the IR report	`model().ir()`
`provenance`	annotated rows; `--semiring`, `--weight`	`provenance(semiring[, weight])`

run --trace[=FILE] writes the event feed to stderr, or to FILE, while the rows go to stdout.

Every command end-to-end over the example scripts, including a script with several queries and one that does not analyse.
find … | xargs relix check exit behaviour.
run --trace produces rows on stdout and events on stderr in one run.

Add the report commands: check, explain, optimize, trace, bundle, ir,…

Done in #36, merged to develop (872e5dd).

Add the report commands: check, explain, optimize, trace, bundle, ir,…