Python: add foreach_get and foreach_set methods to pyrna_prop_array

This allows fast access to various arrays in the Python API.
Most notably, `image.pixels` can be accessed much more efficiently now.

**Benchmark**

Below are the results of a benchmark that compares different ways to
set/get all pixel values. I do the tests on 2048x2048 rgba images.
The benchmark tests the following dimensions:
- Byte vs. float per color channel
- Python list vs. numpy array containing floats
- `foreach_set` (new) vs. `image.pixels = ...` (old)

```
Pixel amount: 2048 * 2048 = 4.194.304
Byte buffer size: 16.8 mb
Float buffer size: 67.1 mb

Set pixel colors:
byte - new - list: 271 ms
byte - new - buffer: 29 ms
byte - old - list: 350 ms
byte - old - buffer: 2900 ms

float - new - list: 249 ms
float - new - buffer: 8 ms
float - old - list: 330 ms
float - old - buffer: 2880 ms

Get pixel colors:
byte - list: 128 ms
byte - buffer: 9 ms
float - list: 125 ms
float - buffer: 8 ms
```

**Observations**

The best set and get speed can be achieved with buffers and a float image,
at the cost of higher memory consumption. Furthermore, using buffers when
using `pixels = ...` is incredibly slow, because it is not optimized.
Optimizing this is possible, but might not be trivial (there were multiple
attempts afaik).

Float images are faster due to overhead introduced by the api for byte images.
If I profiled it correctly, a lot of time is spend in the `[0, 1] -> {0, ..., 255}`
conversion. The functions doing that conversion is `unit_float_to_uchar_clamp`.
While I have an idea on how it can be optimized, I do not know if it can be done
without changing its functionality slightly. Performance wise the best solution
would be to not do this conversion at all and accept byte input from the api
user directly, but that seems to be a more involved task as well.

Differential Revision: https://developer.blender.org/D7053

Reviewers: JacquesLucke, mont29

Fix T54019: copying and linking bugs with custom ID pointer properties.