meson: add src/ folder

This creates a conventional structure of src/ and test/ which helps to
separate top-level files such as meson.build, OWNERS, ...
from implementation files.

Git history of ind

meson: add src/ folder

This creates a conventional structure of src/ and test/ which helps to
separate top-level files such as meson.build, OWNERS, ...
from implementation files.

Git history of individual files is still accessible via e.g.
```
git log --follow -- src/thresholds.hpp
```

Tested: code compiles.

Change-Id: Ifff8b3e70437bc6a25cd6f65afd07d8a563d1a8c
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>

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