SF bug 430991: wrong co_lnotab

Armin Rigo pointed out that the way the line-# table got built didn't work
for lines generating more than 255 bytes of bytecode.  Fixed as he
suggested, plus corresponding changes to pyassem.py, plus added some
long overdue docs about this subtle table to compile.c.

Bugfix candidate (line numbers may be off in tracebacks under -O).