Handle reducible nodes

[zspitz]

• Add option for showing reduced nodes
• If option is selected, call Reduce recursively and create new models for child nodes
• Test against ExpressionTreeToString pathspans of Textual tree renderer, to verify that paths are created correctly

Pending zspitz/ExpressionTreeToString#53

[zspitz]

Reduction options:

• Reduce all, i.e. whenever CanREdcue == true
• Reduce Extension nodes
• Don't reduce