manifest: change manifestctx to not inherit from manifestdict
If manifestctx inherits from manifestdict, it requires some weird logic to
lazily load the dict if a piece of information is asked for. This ended up being
complicated and unintuitive to use.
Let's move the dict creation to .read(). This will make even more sense once we
start adding readdelta() and other similar methods to manifestctx.
(binary:application/octet-stream)