* xmlschemas.c: Added substitution group constraints; changed
  the build of the pre-computed substitution groups. Channeled
  errors during xsi assembling of schemas to the validation
  context. Fixed a big memory leak, which occured when using
  IDCs: the precomputed value of attributes was not freed if
  the attribute did not resolve to an IDC field (discovered
  with the help of Randy J. Ray's schema, posted to the
  xmlschema-dev maling list).