Build Valum with --enable-experimental-non-null

[arteymix]

This will detect possible null assignment and avoid undesired effects.

[arteymix]

This is completed in the ctpl-for-view branch!

I just need to finish the actual CTPL implementation to integrate these changes.

[arteymix]

I have already added a couple of compiler flags, but we cannot yet build with --enable-experimental-non-null in the 0.2 or 0.3 series.

Fixes should be backported into 0.2.