generate Shonkier terms from Pandoc divs and spans

[pigworker]

We should be able to write templates for parts of documents in pandoc markdown. If you write something like

::: {mary="foo(x)"}
This is a paragraph which uses `x`{.mary}.
:::

that should amount to the definition of a shonkier function which computes a Block from an Inline.

Invoking this with

```{.mary}
foo("a template")
```

should amount to generating the paragraph block

This is a paragraph which uses a template.

Implementing this will involve building a ToTerm class whose instances specify how to turn values in various pandoc types into pandoc terms, splicing in fenced shonkier terms as you go.

[gallais]

The current solution only captures code blocks that are variables.
The obvious next step is to allow arbitrary shonkier code to be spliced
in there (thus empowering us to write recursive or even handler templates).

[gallais]

Given that the current strategy does not really work (cf. enum.mary failing
to generate an itemised list), we may want to add support for a simple
templating engine like mustache.

This library claims to implement the spec. The AST is fairly simple so we
could have the same deep integration we have for div & dot and be able to
run scopechecking on the quoted template.