Revert "Refactoring: initialize all layers at once and avoid duplication of slicing height math. #637"

This reverts commit 30d9b2e508471b06e9b081723ef5698c37716c3b.
Revert "New unit test for layer heights"

This reverts commit 0e9d96100b28be2794804fc73c200423b8b9ef8e.
Revert "Add regression test to trap fatal errors during hilbertcurve infill generation. #880"

This reverts commit a633180518ebfe2ec1805aa9d63cc9c9f4c00ccb.
Revert "Bugfix: PlanePath fill patterns were not working anymore. #880"

This reverts commit 177414301139704cb643560a2346c7f0917d5951.
Revert "Bugfix: some holes being closed incorrectly. #858"

This reverts commit 141a8d39898b3b933a422b4b6f70201f356aed54.
Revert "Added failing test case for troubleshooting unexpected filled holes. #858"

This reverts commit 6e6bc746367b4f50dfdac108ce067ac94ff31a30.
Revert "Rename t/stl.t to t/slice.t"

This reverts commit de5b8b9f4d47b2913d411a54b072380882dd9e65.
Revert "Added a couple Clipper tests. #858"

This reverts commit 547e62d0a8b7c8717b9975f7b0eee3dd59d7c832.