fix #4

- fix #3: Inconsistencies while bulk inserting

- some API changes leading to a minor version bump

fix #2: Bug in CloneObject

Added IsIndexCorrupted function to ease checking for ErrIndexCorrupted

Fixed casting bug when a search is made on an empty index

Requires go 1.18 in go.mod

Implement db.AssignIndex allowing to retrieved on indexed values

Added search.ExpectsZeroOrN and changed behavior of search.AssignUniq…

…ue to use ExpectsZeroOrN instead of Expects (prevent from returning ErrNoObjectFound)

Added search.Expects and search.AssignUnique methods + tests

Changes:

 - added Assign and AssignOne utilities + tests
 - added db.AssignAll + tests