These allow to initiate a `NodeTree` from an immutable opaque
sequence of bytes, which could be passed over from Python
(extracted from a `PyBuffer`) or directly mmapped from a file.
Conversely, we can consume
a `NodeTree`, extracting the bytes that express what
has been added to the immutable part, together with the
original immutable part.
This gives callers the choice to start a new Nodetree.
After writing to disk, some would prefer to reread for
best guarantees (very cheap if mmapping), some others will
find it more convenient to grow the memory that was considered
immutable in the `NodeTree` and continue from there.
In `load_bytes`, we anticipate a bit on the file format for
the final version, allowing an offset for fixed data at the
beginning of the file.
This is enough to build examples running on real data and
start gathering performance hints.