Tests

Move Tables.jl and IteratorInterfaceExtensions.jl to extension for faster precompilation #187

Job	Run time
Tests (1, Core) / Tests - Core	6m 10s
Tests (lts, Downstream) / Tests - Downstream	13m 39s
Tests (pre, Core) / Tests - Core	5m 52s
Tests (1, Downstream) / Tests - Downstream	16m 46s
Tests (lts, Core) / Tests - Core	4m 22s
Tests (pre, Downstream) / Tests - Downstream	17m 57s
	1h 4m 46s

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