Precompiled `validation.h`

[mpsijm]

From a discussion with @RagnarGrootKoerkamp on Slack.

The compilation of validation.h starts getting slightly slow. Because this file doesn't regularly change, we can reduce the compile time for the validators by using precompiled headers: https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html

These precompiled headers can probably be stored in /tmp for each problem. If every problem uses a symlink to a shared validation.h, we might even be able to generate one precompiled header file for all these problems, but will probably be a bit messy.