Binary-file-parser

- `RetrieverRef` can now accept an arbitrary number of retrievers to be able to reference data from sub structs.
- `RetrieverRef` can now accept an `int` as part of its arguments to index lists where required
- `RetrieverRef` needs to be given at least one argument
- `RetrieverRef` can now accept `Retriever` or `Combiner` or other `Ref`s as arguments
- `RetrieverCombiner` must be given at least one argument
- `RetrieverCombiner` can now accept `Retriever` or `Ref`s or other `Combiner`s as arguments
- `VersionError` now also subclasses `AttributeError`
- `DefaultValueError` now also subclasses `ValueError`
- Diffing structs now returns a dict of name-value pairs instead of just a list of retrievers.
- Note: This is an expensive operation and should only be used for debug purposes. diff's behaviour is subject to change!
- Added `StrArray8s`, `StrArray16s`, `StrArray32s` and `StrArray64s` datatypes
- Added `Manager` superclass for creating grouped retriever references and functions on them. Use this to provide a more coherent API
for struct modification when the internal struct is messy
- Added docstrings to datatypes, explaining their expected layouts

- For performance reasons, a `parent` reference is no longer available. To synchronise duplicate data in different parts of a file, the data source higher in the struct hierarchy is to be made responsible for performing the synchronisation.
- `RefList` has been removed since parent references are no longer available.
- `struct_ver` is now a read-only property and the functionality for changing a struct's version is no longer available and will be re-added in a future update.
- A `Retriever`'s repeat value is interpreted as follows:
- repeat = 1 (default) means its data type is read once.
- repeat = 0 or > 1 means it is a list of its data type.
- repeat = -1 means the value does not exist and is skipped. (set to `None`)
- repeat set via `set_repeat` will read a list of its data type, even for repeat = 1.
- Note: If a single value is only conditionally present, then its repeat should default to 1, and should be set to -1 dynamically when it is intended to be absent for the desired behaviour. Setting a repeat to 1 dynamically will make it a singleton list!