Use init properties

[jnyrup]

IMPORTANT

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As PolySharp generates IsExternalInit for older runtimes we can use init properties for at minimum internal types

[coveralls]

Pull Request Test Coverage Report for Build 4542024445

• 9 of 9 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.274%

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Totals
Change from base Build 4541862355:	0.0%
Covered Lines:	8760
Relevant Lines:	8988

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💛 - Coveralls