Format code with clang-format

[lucarin91]

I think that can be useful to have a format style for the code base, in order to be more readable and maintainable.
I propose to use clang-format because it is largely used and customizable.

I add the .clang-format that defines the used style. The style is based on the LLVM one but without the sorting of the includes to avoid compilation errors and the reflow of the comments to avoid breaking some ASCII figures in the comments. I also add some other options to inline short if and loops.

In order to run the code formatting, install clang-format (e.g., apt install clang-format) and run the script with ./scripts/format.sh to apply the style to all files.

Note: By default, the LLVM style sets the indentation width to 2 spaces, it is possible to change it to 4 spaces but I think that it makes the code more difficult to read.