Value serialization implemented and tested

Expression serialization still incomplete.
BooleanValue, AbsentValue instead of ReservedConstantValue

Discovered small but important OP_NE case in eval_compare. I don’t know why the problem only appeared after removing ReservedConstantValue, but it suggests to me that the RCV abstraction was mistaken.

Also continue moving toward use of braces of initializers everywhere.
“null” is a specially defined type, not a reserved word
Rename ReservedConstantNode to ReservedWordNode
IdentifierValue fixed to not do implicit evaluation

That was always a mixed metaphor. Now IdentifierNode, part of an Expression, has a clear binding precedence for an identifier.
ObjectValues do need an identifierConversion after all
A quick and incomplete check of Expression serialization
Move to auto-const-ref for almost all for loops

Which is what almost every for-loop wants!
More testing of Expression and a fix to ReservedWordNode::write

Also remove printOutline because YAGNI; ScalarNode goes too.
Implement and test BreakValue; implement Statement serialization

Caught the case of an if-statement without an else-branch defined.

Statement serialization not yet tested.

Illuminated for me the fact that Archetype is not OK with attributes being created on the fly. I like that, although the silent failure is disturbing. That’s why I’d created the /K switch, though.