Preserve usable slot size when RVALUE_OVERHEAD is non-zero #775

Job	Run time
Windows 2025-vs2026 (test-bundled-gems)	23m 0s
Windows 11-arm (btest test-basic test-tool)	14m 24s
Windows 2022 (test-bundled-gems)	15m 53s
Windows 2025-vs2026 (check)	27m 38s
Windows 2022 (check)	22m 31s
Windows result	3s
	1h 43m 29s

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