Basic schema and statement validation

[aew]

• Pass existing validation CATs (including error messages, mostly, code is still overly complex), including valid and invalid examples
• Namespace schema (and statement variable) types for global spec naming
• Define specs at the end after walking the entire tree (rather than as we go). Support validation of recursive object types.
• Support whitespace at the beginning / end of a schema or statement
• Extra eval protection for defining specs only
• Simplify approach to optional vs. required
• Support types for nested lists, including required
• Split schema and statement validation into 2 passes each - one for resolving specs, two for validation rules using them
• Add a defnodevisitor macro to operate exclusively on nodes of a certain type
• Fix some invalid schema scenarios
• Support required for scalars (but not yet for other types)

The error rendering from spec errors to error output matching the CATS is a bit painful and incomplete.

[tendant]

This is great. Thanks a lot.

Please fetch updated master and create new feature branch from master if needed.