Common base class for CEL exceptions, without breaking the existing builtin mappings

[hardbyte]

Follow-up promised when closing #23. Execution errors are mapped to the idiomatic builtin (RuntimeError, TypeError, KeyError, IndexError, ZeroDivisionError, OverflowError) and parse errors to ValueError, which is right for each case but means "did the rule run and fail" needs a five-clause except:

except (RuntimeError, TypeError, KeyError, IndexError, ArithmeticError):

Proposal

Add a cel.CelError hierarchy whose members also inherit from the builtin they replace, so nothing existing breaks:

class CelError(Exception): ...
class CelParseError(CelError, ValueError): ...
class CelRuntimeError(CelError, RuntimeError): ...
class CelTypeError(CelError, TypeError): ...
class CelKeyError(CelError, KeyError): ...
class CelIndexError(CelError, IndexError): ...
class CelZeroDivisionError(CelError, ZeroDivisionError): ...
class CelOverflowError(CelError, OverflowError): ...

except TypeError keeps working; except cel.CelError catches everything CEL raised; except cel.CelParseError separates a bad rule from a failed check.

Implementation notes

PyO3's create_exception! only takes a single base, so define the classes in a small python/cel/exceptions.py and have map_execution_error_to_python look them up from the module once (a GILOnceCell<Py<PyType>> per class) and raise with PyErr::from_type. Add them to cel.pyi and to the error-handling how-to, whose current table becomes the mapping between the two hierarchies.