Add tree syntax

[bvssvni]

One idea is to add tree syntax as a way of making it simpler to create structured data.

ancient_egypt {
  early_dynastic_period,
  old_kingdom_era,
}

This is the same as:

(early_dynastic_period, ancient_egypt)
(old_kingdom_era, ancient_egypt)

[bvssvni]

Related to #164

An idea is to combine tree syntax with namespaces:

ns ancient_egypt {
  ::early_dynastic_period,
  ::old_kingdom_era,
}

This is the same as:

(ancient_egypt::early_dynastic_period, ancient_egypt)
(ancient_egypt::old_kingdom_era, ancient_egypt)