Make children formal, remove initializer parameter

children used to be an initializer parameter of Node and most of its
abstract subclasses, declared default. After some discussion in #35,
@gavinking pointed out that it should be formal instead, and be removed
as an initializer parameter. This commit does that, solving the problem
of refinement and usage of default members in the initializer section.

Closes #35.
Documentation for GroupedExpression.innerExpression

Not “good” documentation or “better” documentation. Just documentation.
source-gen: Update for #35