Lightdb

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Added
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- Created `models.py` for model implementation
- Created `fields.py` for field implementation and validation
- Created `query.py` for executing queries on model data
- Created `exceptions.py` to define custom exceptions related to models
- Added tests for LightDB and models
- Updated `README.md` with documentation on using models
- Added an example of using models in `examples/models_usage.py`

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- Rename `LightDB.py` -> `core.py`
- Update license

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Added
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- Rewrite code and docstrings
- Rewrite README
- Added tests

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Fixed
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- Fixed error while getting non string key

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Fixed
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- Fixed encoding error

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Added
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- Added methods: `set_key()`, `get_key()`, `pop_key()`