Add fix-includes Makefile target

Basically

ninja -C build iwyu > iwyu.out
(cd build && fix_includes.py < ../iwyu.out)

• should place iwyu.out in a temp dir and delete afterwards
• create tools/fix_includes shell script?
• run clang-format afterwards

changed the description

mentioned in merge request !70 (merged)

closed with merge request !70 (merged)

mentioned in commit b3ecbb1e

closed with commit 484adf89

mentioned in commit 484adf89