Dataiter

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* `dt`: Handle all NaT input
* Migrate from `setup.py` to `hatch` and `pyproject.toml`

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* `Vector.as_datetime`: Add `precision` argument
* `Vector.concat`: New method
* `Vector.sort`: Fix sorting object vectors

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* `DataFrame`: Fix column and method name clash errors in certain operations
* `dt.replace`: Allow vector arguments the same length as `x`

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* `DataFrame.count`: New method, shorthand for
`data.group_by(...).aggregate(n=di.count())`
* `Vector.rank`: Handle empty and all-NA vectors

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* `USE_NUMBA_CACHE`: New option, read from environment variable
`DATAITER_USE_NUMBA_CACHE` if exists, defauls to `True`
* Fix a possible issue with Numba caching

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* Use `numba.extending.overload` instead of the deprecated
`numba.generated_jit`