[dtype] Support Unions

[sneakers-the-rat]

fix: https://github.com/p2p-ld/numpydantic/issues/19

Much like how we support the labor movement, we support the evaluation of union dtypes

Also:

• Perf improvements from not iterating over every element of an array
• Renaming array to value in all serialized forms

[coveralls]

Pull Request Test Coverage Report for Build 11008989398

Details

• 71 of 72 (98.61%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.06%) to 99.169%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/numpydantic/serialization.py	20	21	95.24%
<!--	Total:	71	72	98.61%	-->

Totals
Change from base Build 11005418678:	-0.06%
Covered Lines:	1194
Relevant Lines:	1204

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💛 - Coveralls