Code improvements

[eyip002]

• Improved support for macOS serial port setup.
• More efficient lookup for the state data structure: added functions to return a track element's index into their respective state data structure. Avoids the need to iterate over the state and to compare string values.
• Other minor improvements.

[eyip002]

@nicolasgross

Looks good to me. However, I was wondering what the purpose for these new functions is, since they are not called anywhere?

These functions are called in the program that uses libbidib. See here for an example: https://github.com/uniba-swt/swtbahn-cli/blob/8a623c7fb3df240d2649b504ca1b5387d665602b/server/src/handler_controller.c#L66-L77