use map files instead of output from nm as a source of symbols and sizes

[awujek]

In my project I noted that using the map file generated during the linking is much mode accurate than using output of nm. nm does not provide size of the data placed in .rodata section which does not have own symbol e.g. text strings.

In my case puncover reports the size as ~75KB of code + 11.5KB of static. The size command reports ~125KB.

I'm aware that using map file has some disadvantages (like sometimes only elf file is available) and will require significant changes in the puncover, but I think it is worth to be at least considered.