Add ability to iterate over key/value pairs in a map

[jgm]

Something like

$forpairs(meta)$
- $meta.key$: $meta.value$
$endforpairs$

And when forpairs is applied to a regular array, key should be set to the index (starting with 1). This would allow us to create numbered lists, for example:

$forpairs(items)$
$item.key$. $item.value.name$ ($item.value.price$)
$endforpairs$

An alternative approach would be to provide some general mechanism for turning a map into an array with key/value objects:

$for(meta/pairs)$
$it.key$ = $it.value$
$endfor$

$meta/pairs:partial()$

Thinking ahead, the / could perhaps be reserved for filters that alter a variable:

$date/iso8601$
$name/uppercase/underscores$

@mb21