Nested/Inner Classes
====================

This is very useful for:
* small record types
    for example in:
    - multiple return values
    - extensible alternative to function parameters/return values
    - in data structures, e.g. lists, maps.
    - to avoid repetition.
* builder classes
* wrappers

Should it be allowed for all, or only for e.g. records?
- functions inside them will only be allowed to access data that belongs
  to the nested class.

Should it be possible to expose/export inner classes?
- within the module?
- outside of the module?


Syntax
------
Should it implicitly be a `data` block?
What keyword(s) to use?
- `local class`
- `inner class`
- `nested class`
- `nested`

Example:

    nested Point
        int x
        int y
    end

    data
        Point p1
        Point p2
    end

Or

    # or
    #nested data Point
    #inner data Point
    nested class Point
        int x
        int y
    end

Or (best?)

    nested Point
        data
            int x
            int y
        end
    end

    data
        Point p1
        Point p2
    end

Or

    # In order to give good error messages when `class N` is placed the
    # beginning of N.slul, let's use some distinctive keyword sequence
    # instead of just `class`:
    local class Point
        data
            int x
            int y
        end
    end

    data
        Point p1
        Point p2
    end