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ARROW-11913: [Rust] Improve performance of StringBuilder by delaying bitmap creation

`PrimitiveBuilder<u8>` was performing a lot of unnecessary bit operations.
One operation for each byte of each string one appends.

Delaying the creation of null bitmap builder for `PrimitiveBuilder`
significantly improves performance of `StringBuilder`. The optimization is
also generally useful as the null bitmap is not stored if resulting
arrays do not contain nulls.

Results from the added benchmark on my machine are:

```
bench_primitive/bench_string
                        time:   [6.8971 ms 6.9123 ms 6.9335 ms]
                        thrpt:  [937.48 MiB/s 940.35 MiB/s 942.43 MiB/s]
                 change:
                        time:   [-49.585% -49.395% -49.204%] (p = 0.00 < 0.05)
                        thrpt:  [+96.867% +97.610% +98.355%]
                        Performance has improved.
```

Closes #9662 from ilya-biryukov/optstr

Authored-by: Ilya Biryukov <iu.biryukov@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>