constant-time directory lookup

[davidsarah]

Currently to look up an entry in a directory, it is necessary to download the whole file backing the directory, then decode it to find the correct child. This takes linear time in the number of entries.

Suppose that the secret cryptovalue of each child were derived from the cap of the parent directory and the child name. Then, provided that the metadata is not needed, it is possible to obtain the child directly rather than via the parent. (If that child does not exist, we can't distinguish that from missing shares, but that isn't necessarily a problem.)

Note that this makes directories more useful for some database-like applications, rather than just as filesystem directories per-se.