om: add namespace support to enumerations in domain formatter
sml: add support for ostream and ostringstream
sml: add an indexer service to sml model and generate it

- will be used by the file formatters in OM. added an interface to
  make testing easier (e.g. mock the indexer)
diagrams: add create a context class to om

- context will be responsible for storing top-level formatter state for the
  duration for the duration of a format call.