ObjStorage is now using bytearrays for its keys instead of hex strings

functools.wraps preserves the wrapped function's metadata which makes
navigation and interactive use nicer

Nested revisions can now be handled through directory_entry_rev

Those tables are useful to let a directory point to another revision,
for instance when using submodules or for tarball in tarball.

SQL: use bytea for checksum types

as a side effect, change the API of storage.add_content to require more
complete dictionaries. In particular, dictionaries passed there need to have an
explicit "length" key

it can create temporary tables that mimick any existing table

we had a default, but that is not enough to ensure an (explicit) NULL will not
be inserted