Dnfile

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* BREAKING CHANGE: structure attributes no longer exist by default
* BREAKING CHANGE: objects' attributes always exist, but may be None
* BUGFIX: use last stream if multiple of same name
* CI: added mypy type checking
* when duplicate stream names, behave like runtime and use last one for shortcuts
* add user_strings shortcut
* able to access MetaDataTables like a 0-based list, with square brackets
* added use of logging module for warnings
* better type hints for IDEs
* more better source comments
* more tests

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* bugfix: row indices parsed in structures are one-based, not zero-based
* bugfix: TypeDefRow was not parsing Extends coded index
* bugfix: incorrect BLOBS_MASK and add EXTRA_DATA skip if flag set
* added CI using github workflow
* added tests and submodule dnfile-testfiles
* added style consistency using pycodestyle and isort
* added more examples
* parse MetaData tables' list-type indexes into lists of MDTableRow objects

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* bugfix: Metadata Table indexes (i.e. indexes into other tables) were off by one

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* bugfix: coded index always None

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* bugfix: improper data length check

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* bugfix: referenced wrong object
* parse utf-16 strings in US stream