Function merge_filesystems

async fn merge_filesystems(
    source: &dyn FileSystem,
    destination: &dyn FileSystem,
) -> Result<(), FsError>

Merge the contents of one filesystem into another.

NOTE: merging is a very expensive operation, since it requires copying many files in memory, even if the underlying files are immutable and mapped through mmap or similar mechanisms. Merging should be avoided when possible.