Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

UnitTests: Remove redundant `virtual` specifier

Fulfilling a certain six-year-old todo.

Using the updated rangeset library

Using GTEST_SKIP instead of just returning from the function shows that
a test was skipped in the test summary. If GTEST_SKIP is called the rest
of the function won't be run, just like with the return.

GTEST_SKIP wasn't available until gtest 1.10, and we updated to 1.12 in
597f8f1b874bf93854ae178795c55117f680e457.

This fixes a bunch of DEBUG_ASSERTs in the unit tests.