make disjoint pool a C structure #4863

Job	Run time
DocsBuild / Docs build	1m 1s
CodeChecks / Basic code checks	1m 18s
Fast builds / Fast builds (windows-latest, ON, OFF)	1s
Fast builds / Fast builds (windows-latest, OFF, OFF)	2m 12s
Fast builds / Fast builds (ubuntu-latest, ON, -DCMAKE_BUILD_TYPE=Release -DUMF_BUILD_BENCHMARKS=ON -DUMF_BUILD_...	4m 28s
Fast builds / Fast builds (ubuntu-latest, OFF, -DCMAKE_BUILD_TYPE=Release -DUMF_BUILD_BENCHMARKS=ON, OFF)	2m 35s
Fast builds / Fast builds (ubuntu-latest, ON, -DCMAKE_BUILD_TYPE=Release, ON)	3m 0s
Fast builds / Fast builds (ubuntu-20.04, ON, -DCMAKE_BUILD_TYPE=Release, ON)	1s
Basic builds	0s
DevDax	0s
MultiNuma	0s
QEMU	0s
Sanitizers	0s
CUDA	0s
L0	0s
Benchmarks	0s
ProxyLib	0s
CodeQL	0s
Valgrind	0s
Trivy	0s
Coverage	0s
Coverage_partial	0s
	14m 36s

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