Return no_export instead of falling through.

Otherwise, the value is assigned to the dict and then future lookups of
the extern labels fail.
Fix, again, the auto-litflag bitmasks.
Remove usage of signed shorts.

Except for bitwise stuff and arbitrary strings, only encode using
signed integers and doubles.
Replace args list in relocations tables with bitmask.

Same data, no variable number of reads to get it. This is not the same
format as litflags, as the flags' bits correspond 1-to-1 with the
instructions' operands.
Add externs_data and externs relocation tables to unit output.
Add document describing the current bytecode format.