Queries in documents

A card can carry a query. Rendering the card runs the query against the vault and writes the answer beneath it, so a list that would otherwise be maintained by hand is computed instead.

An hql#eval block

Write the query in a fenced block whose info string is hql#eval:

```hql#eval
cards
| filter(c => c.metadata.status == "todo")
| sort(by = c => c.title)
| map(c => c.title)
| table
```

Rendering

hql render runs every such block in a document and prints the document with each answer transcluded as an hql#result block:

hql --vault examples/birds/wiki render examples/birds/wiki/field-notes.hmd

The field notes card in the birds example computes a summary of its own observations:

```hql#eval
sightings = ["barn-owl", "european-robin", "barn-owl", "common-raven"]
occurrences : Collection<String> = sightings
species = Set(sightings)

{
    observations: size(occurrences),
    barn_owls: count(occurrences, "barn-owl"),
    species: size(species),
    saw_robin: contains(species, "european-robin"),
}
```

```hql#result
key           value
------------  -----
barn_owls     2
observations  4
saw_robin     true
species       3
```

Without --write, rendering prints and changes nothing. --write <file> writes the rendered document, replacing each previous hql#result block rather than appending another.