Add namespaces

[bvssvni]

For example, when structuring data about ancient Egypt, I don't want to say explicitly that it is about ancient Egypt. However, when I combine this data with ancient Greece, there can be naming collisions.

[bvssvni]

Idea for syntax:

ns ancient_egypt {
  (::early_dynastic_period, before'(::old_kingdom_era))
}

This is equal to:

(ancient_egypt::early_dynastic_period, before'(ancient_egypt::old_kingdom_era))

[bvssvni]

An idea for use syntax:

use ancient_egypt::early_dynastic_period as edp

(edp::first_dynasty, ...)