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ARROW-11291: [Rust] Add extend to MutableBuffer (-20% for arithmetic, -97% for length)

# Rational

Rust forbids safely accessing uninitialized memory because it is undefined behavior. However, when building `Buffer`s, it is important to be able to _write_ to uninitialized memory regions, thereby avoiding the need to write _something_ to it before using it.

Currently, all our initializations are zeroed, which is expensive. #9076 modifies our allocator to allocate uninitialized regions. However, by itself, this is not useful if we do not offer any methods to write to those (uninitialized) regions.

# This PR

This PR is built on top of #9076 and introduces methods to extend a `MutableBuffer` from an iterator, thereby offering an API to efficiently grow `MutableBuffer` without having to initialize memory regions with zeros (i.e. without `with_bitset` and the like).

This PR also introduces methods to create a `Buffer` from an iterator and a `trusted_len` iterator.

The design is inspired in `Vec`, with the catch that we use stable Rust (i.e. no trait specialization, no `TrustedLen`), and thus have to expose a bit more methods than what `Vec` exposes. This means that we can't use that (nicer) API for trustedlen iterators based on `collect()`.

Note that, as before, there are still `unsafe` uses of the `MutableBuffer` derived from the fact that it is not a generic over a type `T` (and thus people can mix types and grow the buffer in unsound ways).

Special thanks to @mbrubeck for all the help on this, originally discussed [here](https://users.rust-lang.org/t/collect-for-exactsizediterator/54367/6).

```bash
git checkout master
cargo bench --bench arithmetic_kernels
git checkout length_faster
cargo bench --bench arithmetic_kernels

git checkout 16bc7200f3baa6e526aea7135c60dcc949c9b592
cargo bench --bench length_kernel
git checkout length_faster
```

```
Switched to branch 'length_faster'
  (use "git pull" to merge the remote branch into yours)
   Compiling arrow v3.0.0-SNAPSHOT (/Users/jorgecarleitao/projects/arrow/rust/arrow)
    Finished bench [optimized] target(s) in 1m 02s
     Running /Users/jorgecarleitao/projects/arrow/rust/target/release/deps/arithmetic_kernels-ec2cc20ce07d9b83
Gnuplot not found, using plotters backend
add 512                 time:   [522.24 ns 523.67 ns 525.26 ns]
                        change: [-21.738% -20.960% -20.233%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  9 (9.00%) high mild
  3 (3.00%) high severe

subtract 512            time:   [503.18 ns 504.93 ns 506.81 ns]
                        change: [-21.741% -21.075% -20.308%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 4 outliers among 100 measurements (4.00%)
  2 (2.00%) high mild
  2 (2.00%) high severe

multiply 512            time:   [508.25 ns 512.04 ns 516.06 ns]
                        change: [-22.569% -21.946% -21.305%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 1 outliers among 100 measurements (1.00%)
  1 (1.00%) high mild

divide 512              time:   [1.4711 us 1.4753 us 1.4799 us]
                        change: [-24.783% -23.305% -22.176%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 5 outliers among 100 measurements (5.00%)
  2 (2.00%) high mild
  3 (3.00%) high severe

limit 512, 512          time:   [373.47 ns 377.76 ns 382.21 ns]
                        change: [+3.3055% +4.4193% +5.5923%] (p = 0.00 < 0.05)
                        Performance has regressed.

add_nulls_512           time:   [502.94 ns 504.51 ns 506.28 ns]
                        change: [-24.876% -24.299% -23.709%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 2 outliers among 100 measurements (2.00%)
  2 (2.00%) high severe

divide_nulls_512        time:   [1.4843 us 1.4931 us 1.5053 us]
                        change: [-22.968% -22.243% -21.420%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 24 outliers among 100 measurements (24.00%)
  15 (15.00%) low mild
  1 (1.00%) high mild
  8 (8.00%) high severe
```

Length (against the commit that fixes the bench, `16bc7200f3baa6e526aea7135c60dcc949c9b592`, not master):

```
length                  time:   [1.5379 us 1.5408 us 1.5437 us]
                        change: [-97.311% -97.295% -97.278%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 12 outliers among 100 measurements (12.00%)
  1 (1.00%) low severe
  4 (4.00%) low mild
  3 (3.00%) high mild
  4 (4.00%) high severe
```

Closes #9235 from jorgecarleitao/length_faster

Lead-authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Co-authored-by: Matt Brubeck <mbrubeck@limpet.net>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>