Release 0.175.0

This release contains two breaking changes to recently added
features.

Change to jsonpath::get function

• The return value for jsonpath::get has been changed from a
  pointer to the selected JSON value, or null if not found, to a
  std::pair<Json*,bool>, where the bool component indicates
  whether the get operation succeeded.

Change to new jsonschema classes and functions introduced in 0.174.0:

• The overload of json_schema<Json>::validate that takes a callback
  must now be passed a lambda that returns walk_result::advance or
  walk_result::abort. This supports early exit from validation.

Note that this change does not affect the legacy pre-0.174.0 jsonschema
classes and functions (make_schema, json_validator).

Enhancement to jsonschema library:

• New json_schema member function walk for walking through the schema.