Add annotated copy of lab4.asm to data/
Miscellaneous fixes and improvements

    - Remove tag_cgcontext::lastlabel (unused)
    - Fix OP_POS (off by one error)
    - Add PUSH/POP support
    - Remove sys_trap_{halt,out,puts} (unused)
    - Add PUTSP, GETC, IN as compiler-recognized traps
    - Fix decimal number scanning
    - Output obj file in network byte order (for debugging)

Known Issues:
    - not quite there yet
Successful compiler code generation for Great Justice!!!

    Changes:
        - Fixed AA_IMM_NBITS (off by one)
        - Fixed BR_CC_NEGATIVE and BR_CC_POSITIVE (swapped values)
        - Added lab4.asm (source for lab4)
        - Added lab4.o3 (assembled binary)
        - Added lab4.sym (symbol file)
        - Added copy of lab4 with address labels
        - Added by-hand disassembly of lab4.o3
    Caveats:
        - Everything is in network byte order (which is probably
          The Right Thing, but is not compatible with the simulator)
        - Only the instructions present in lab4.asm have been
          tested.
Merge branch 'codegen' into codegen-linux