When importing from subdirectories, the line info used in `UnusedImport`
warning would be the `/` node and not the actual module node. More
obvious with grouped imports where all unused imports would show the
same column

Fix is to just use the last child node for infixes when getting the line
info
* clean up some test categories
* mention exact slice issue
* magics into system
* move trangechecks into overflow
* move tmemory to system
* try fix CI
* try fix CI
* final CI fix