Remove generic builder pattern from element metadata create (Part 2): AddChild

[twsouthwick]

This replaces AddChild<T> with a non-generic version. Brings the time down another couple percentage:

Before

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Create	39.82 μs	0.756 μs	0.708 μs	6.1035	0.2441	51.52 KB
CreateNoSave	39.16 μs	0.620 μs	0.550 μs	6.1035	0.2441	51.52 KB
ReadFile	868.19 μs	17.059 μs	33.672 μs	29.2969	3.9063	255.22 KB

After

Method	Mean	Error	StdDev	Gen0	Gen1	Allocated
Create	41.71 us	0.776 us	0.762 us	6.1035	0.2441	51.52 KB
CreateNoSave	40.32 us	0.729 us	0.780 us	6.1035	0.2441	51.52 KB
ReadFile	847.70 us	16.828 us	29.474 us	29.2969	3.9063	255.22 KB

[github-actions]

Test Results

    70 files  ±0      70 suites  ±0   1h 7m 30s ⏱️ - 1m 30s
 2 041 tests ±0   2 038 ✅ ±0   3 💤 ±0  0 ❌ ±0 
32 418 runs  ±0  32 382 ✅ ±0  36 💤 ±0  0 ❌ ±0 

Results for commit be53c40. ± Comparison against base commit 89cd570.