ec: expose low-level point arithmetic #1037

Job	Run time
Tests (4.14.2, ubuntu-latest)	2m 9s
Tests (4.14.2, macos-latest)	2m 56s
Tests with OCaml 5 (5.2.1, macos-latest)	2m 27s
Tests with OCaml 5 (5.2.1, ubuntu-latest)	1m 26s
	8m 58s

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