Package exports

[mortenpi]

Currently, the package does not export anything, so everything has to be explicitly included. We probably want to export some (or all) of the following things:

• @ast macro
• Node type
• Functions for querying and mutating trees (next, insert_after! etc).
• Abstract element types (AbstractElement, AbstractInline etc).
• Concrete element types (note: Text is ambiguous with a Base export; but the @ast macro does not actually need the Text() method, so it could remain unexported).