* `strings`: Remove unused `hash32_utf8` function
* `strings`: Make `hash_utf8` and `decode_Utf8Str` private
* `strings`: Reorganize private functions
* 'strings': Raise error when non-string/-int types are passed to functions that don't accept them
* `strings`: Add `items()` method, add type hints, remove unused methods, restrict inputs to specific types, reorganize methods
* `Morphology`: Use `StringStore.items()` to enumerate features when pickling
* `test_stringstore`: Update pre-Python 3 tests
* Update `StringStore` docs
* Fix `get_string_id` imports
* Replace redundant test with tests for type checking
* Rename `_retrieve_interned_str`, remove `.get` default arg
* Add `get_string_id` to `strings.pyi`
Remove `mypy` ignore directives from imports of the above
* `strings.pyi`: Replace functions that consume `Union`-typed params with overloads
* `strings.pyi`: Revert some function signatures
* Update `SYMBOLS_BY_INT` lookups and error codes post-merge
* Revert clobbered change introduced in a previous merge
* Remove unnecessary type hint
* Invert tuple order in `StringStore.items()`
* Add test for `StringStore.items()`
* Revert "`Morphology`: Use `StringStore.items()` to enumerate features when pickling"
This reverts commit 1af9510ceb.
* Rename `keys` and `key_map`
* Add `keys()` and `values()`
* Add comment about the inverted key-value semantics in the API
* Fix type hints
* Implement `keys()`, `values()`, `items()` without generators
* Fix type hints, remove unnecessary boxing
* Update docs
* Simplify `keys/values/items()` impl
* `mypy` fix
* Fix error message, doc fixes
* Draft out initial Spans data structure
* Initial span group commit
* Basic span group support on Doc
* Basic test for span group
* Compile span_group.pyx
* Draft addition of SpanGroup to DocBin
* Add deserialization for SpanGroup
* Add tests for serializing SpanGroup
* Fix serialization of SpanGroup
* Add EdgeC and GraphC structs
* Add draft Graph data structure
* Compile graph
* More work on Graph
* Update GraphC
* Upd graph
* Fix walk functions
* Let Graph take nodes and edges on construction
* Fix walking and getting
* Add graph tests
* Fix import
* Add module with the SpanGroups dict thingy
* Update test
* Rename 'span_groups' attribute
* Try to fix c++11 compilation
* Fix test
* Update DocBin
* Try to fix compilation
* Try to fix graph
* Improve SpanGroup docstrings
* Add doc.spans to documentation
* Fix serialization
* Tidy up and add docs
* Update docs [ci skip]
* Add SpanGroup.has_overlap
* WIP updated Graph API
* Start testing new Graph API
* Update Graph tests
* Update Graph
* Add docstring
Co-authored-by: Ines Montani <ines@ines.io>