Editable sections data

[zygzagZ]

My assignment was to convert 32-bit obj files to 64-bit and vice-versa, which required fiddling with ELF symbols, relocations, adding new ones, modifying data etc.

Did not find suitable tool (did not look too long, too), decided to go with rbelftools and implement what lacked.

Idea:

• instead of patches keep full sections data
• allow rebuilding sections
• allow rebuilding whole elf files
• try to keep lazy-loading, even though when saving all data must be loaded anyway
• keep backward compatibility

All in all, I figure it is not that bad of a code.

Assumes #62 to work.

Part of #52.