PoC schema for metaconstraints.

Changed metapath type to indicate a difference between a Metapath that can be compiled vs a string that is to be processed in a different way. Also removed extra unnecessary ConstraintsContainerType.
Added support for flag and field instances to provide a different default value than the definition.
Reorganized the schema to group constraint types and groups.
Added support for let statements in constraints.
Added support for choice groups as a potential solution for resolving #228.
Fixed some minor content defects.
Added a draft Metaschema Metaschema in schema/metaschema/metaschema-metaschema.xml.
Adjusted Metaschema choice-group to allow for minimum cardinalities of 1 and a default unlimited max cardinality.
Created a draft Metaschema module (metaschema-module-metaschema.xml, describing the Metaschema module model.