Reduce archetype size by storing layouts in a growing slice

[mlange-42]

Use a growing slice to hold archetype mem layouts, instead of a fixes-size array.

Reduces archetype size from 4 kB to 88 B, plus the heap-allocated slice. The slice starts with size 16 (i.e. 16 component types) and grows in steps of 16 up to 256.

[coveralls]

Pull Request Test Coverage Report for Build 7515866108

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 7514965557:	0.0%
Covered Lines:	4871
Relevant Lines:	4871

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💛 - Coveralls