Now that the Lemmy API returns nice timestamps, it's all done automatically by Go's JSON package.

This commit adds tests too as it's starting to get a bit trickier.

Major rework which keeps Stat, Read and ReadDir away from the filesystem tree and more to the file. The fs package implements extra functionality that doesn't really have anything to do with the core package. It also does not need to know how the core lemmy package is implemented to do anything, so may as well enforce the boundary explicitly.

This keeps things simpler