TODO
====
  - LDS and STS instructions don't address I/O
  - ATmega8 CALL instruction is bogus
  - recursion - sigsegv
  - Conditional assembly with forward referenced label on .DW directive generates incorrect code
  - AVRA complains about "((str_start_%)<<1)" beeing invalid
  - if last line in include is a remark line, it is written twice in the listing
  - Printing of diagnostic messages with hexademical numbers is inconsistent
    (some are %x; others %X)
