Added exe program and fixed linker

  - "ld" serves one simple purpose: gluing object files together
    in the correct way
  - "exe" serves another simple purpose: turning an object file
    into an executable (currently only "flat binary" supported)

Fixes:
  - fixed object file footer generation in linker (additional 0xFF
    at end of file because of EOF character)
Replaced undefined size integers with fixed size ints in linker
Overhauled object file format

  - Shrinked footer (removed unnecessary data)
  - Sections have to be in that order
  - Export symbol table comes first, it doesn't have to be altered by
    the linker