This branch propose to avoid clones in append by storing offset and size
in previous overlay depth.
That way on rollback we can just truncate and change size of existing
value.
To avoid copy it also means that :

- append on new overlay layer if there is an existing value: create a
new Append entry with previous offsets, and take memory of previous
overlay value.
- rollback on append: restore value by applying offsets and put it back
in previous overlay value
- commit on append: appended value overwrite previous value (is an empty
vec as the memory was taken). offsets of commited layer are dropped, if
there is offset in previous overlay layer they are maintained.
- set value (or remove) when append offsets are present: current
appended value is moved back to previous overlay value with offset
applied and current empty entry is overwrite (no offsets kept).

The modify mechanism is not needed anymore.
This branch lacks testing and break some existing genericity (bit of
duplicated code), but good to have to check direction.

Generally I am not sure if it is worth or we just should favor
differents directions (transients blob storage for instance), as the
current append mechanism is a bit tricky (having a variable length in
first position means we sometime need to insert in front of a vector).

Fix #30.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: EgorPopelyaev <egor@parity.io>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Bastian Köcher <info@kchr.de>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>