Add different item backends via msgspec

[stephantul]

This PR adds different item backends, implemented via msgspec.

[stephantul]

Closes #64

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.48718% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
vicinity/items.py	77.77%	4 Missing ⚠️
vicinity/vicinity.py	77.77%	4 Missing ⚠️

Files with missing lines	Coverage Δ
vicinity/datatypes.py	100.00% <100.00%> (ø)
vicinity/items.py	77.77% <77.77%> (ø)
vicinity/vicinity.py	92.81% <77.77%> (-4.87%)	⬇️

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