Otherwise, the `GIX_VERSION` environment variable is not available at build time,
which can lead to runtime errors.

The buffer will now be previded from the free-list of the repository.

…st`.

That way, one day we can turn this type into a compatible one which produce
different kinds of hashes as well.

With it it's easy to alter existing trees or build entirely new ones,
efficiently.

…y of entry names.

Previously, it would allow null-bytes in the name which would corrupt the written tree.
Now this is forbidden.
For some reason, it disallowed newlines, but that is now allowed as validation is should
be handled on a higher level.

It's faster throughout the board.

```
❯ cargo bench -p [email protected] --bench edit-tree
   Compiling gix-object v0.44.0 (/Users/byron/dev/github.com/Byron/gitoxide/gix-object)
   Compiling gix-pack v0.53.0 (/Users/byron/dev/github.com/Byron/gitoxide/gix-pack)
   Compiling gix-odb v0.63.0 (/Users/byron/dev/github.com/Byron/gitoxide/gix-odb)
    Finished `bench` profile [optimized] target(s) in 5.97s
     Running benches/edit_tree.rs (target/release/deps/edit_tree-6af6651a1c453a05)
Gnuplot not found, using plotters backend
editor/small tree (empty -> full -> empty)
                        time:   [2.5972 µs 2.6019 µs 2.6075 µs]
                        thrpt:  [3.8351 Melem/s 3.8434 Melem/s 3.8503 Melem/s]
                 change:
                        time:   [-32.618% -32.355% -32.038%] (p = 0.00 < 0.05)
                        thrpt:  [+47.142% +47.831% +48.409%]
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  13 (13.00%) high mild
  1 (1.00%) high severe
editor/deeply nested tree (empty -> full -> empty)
                        time:   [8.2019 µs 8.2079 µs 8.2145 µs]
                        thrpt:  [5.5998 Melem/s 5.6043 Melem/s 5.6084 Melem/s]
                 change:
                        time:   [-33.517% -33.377% -33.246%] (p = 0.00 < 0.05)
                        thrpt:  [+49.804% +50.099% +50.415%]
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  8 (8.00%) high mild
  5 (5.00%) high severe

cursor/small tree (empty -> full -> empty)
                        time:   [2.6911 µs 2.6935 µs 2.6961 µs]
                        thrpt:  [3.7090 Melem/s 3.7127 Melem/s 3.7160 Melem/s]
                 change:
                        time:   [-33.881% -33.546% -33.225%] (p = 0.00 < 0.05)
                        thrpt:  [+49.757% +50.480% +51.242%]
                        Performance has improved.
Found 14 outliers among 100 measurements (14.00%)
  4 (4.00%) high mild
  10 (10.00%) high severe
cursor/deeply nested tree (empty -> full -> empty)
                        time:   [1.3616 µs 1.3631 µs 1.3649 µs]
                        thrpt:  [33.703 Melem/s 33.747 Melem/s 33.783 Melem/s]
                 change:
                        time:   [-40.063% -39.675% -39.234%] (p = 0.00 < 0.05)
                        thrpt:  [+64.566% +65.769% +66.843%]
                        Performance has improved.
Found 20 outliers among 100 measurements (20.00%)
  18 (18.00%) high mild
  2 (2.00%) high severe
```

… way to get a fully-owned `Tree`

Create a tree editor using `Tree::edit()` or `Repository::edit_tree(id)`.

An implementation of `Header`, `Write` and `Find`, that can optionally
write everything to an in-memory store, and if enabled, also read
objects back from there.

That way it can present a consistent view to objects from two locations.

The default object database changed to a version that allows to
keep objects in memory. This needs a mutable `Repository` instance
to setup.