Compiler output and stderr not taken into account for cache size computation

[siu]

The size of the output.txt and stderr.txt files are not taken into account when computing the size of the artifacts repository. This allows the cache to be bigger than configured.

This is a minor issue as those files are not huge.

[frerich]

Indeed - it appears that in CompilerArtifactRepository.clean, the code should maybe also query the file size of all stored artifacts. This would become especially relevant in case we add support for caching external debug information (see #30) since the .pdb files can become rather big.