Keep the top-level directory clean of source files

[ruuda]

Newcomer suggestion: move all of the source files into an src/ directory. This would make it easier to navigate the source tree and to locate other files such as the readme (both locally and on GitHub); to check whether there is something like a Makefile or CMakeLists.txt or autogen.sh (because the first thing you do after cloning is of course running make, not reading the readme).

[jan-wassenberg]

We heeded this advice in JPEG XL, which will soon be open sourced and supersede this repo.