runcom -> rc in cache benchmark

This renaming was done before merging the cache, but was forgotten in
`benchmakrs/cache.ipynb` since it's not run automatically anywhere.
Fix sign error in cache performance test.
Store cache files as NCO.

Store the pickle part and array part of the cache data in a single
"Nengo cache object" file. That reduces overhead due to a lot of small
pickle files. Also, it ensures some more consistency in the cache
because it is not possible anymore that only the pickle or the array
file gets lost. That again frees us from checking for orphans which
gives a slightly better performance.
Count file system overhead towards cache size.
Remove unused argument in DecoderCache.get_size
Distribute cache files in subfolders.

File system performance degrades with too many files in one directory.
Remove decoders cached before cache changes.

We do this by checking if a versioned "legacy file" exists and matches
the current legacy version; if not, we'll do a sweep for old cached
files. This logic should be updated if we ever change the format
in which we store cache files.