Speedup `_cg_couple_sparse` a bit #554

Job	Run time
Lint Rust code (stable, x86_64-unknown-linux-gnu)	6s
check examples / benchmarks	1m 42s
ubuntu-22.04 / rust stable / static C library	7m 23s
ubuntu-22.04 / rust stable / release valgrind	7m 49s
ubuntu-22.04 / rust 1.74 / cmake 3.16	7m 16s
macos-14 / rust stable	4m 14s
windows-2019 / rust stable	12m 11s
	40m 41s

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