rust: in zstd decompression, avoid a useless vec initialization

rust: speed up zstd decompression by re-using the decompression context Admittedly, zstd is already pretty fast, but this change makes it a bit faster yet: it saves ~5% of time it takes to read our large repo. The actual motivating use case is treemanifest: in treemanifest we end up reading *lots* of small directories, and many of them need decompression, and there the saving for [rhg files] is >10%. (which also seems unreasonable, we should probably keep things uncompressed more)

store: rename `topfiles` to `top_entries` The method is now returning StoreEntries let us rename the method for clarity.