Fix annoying bugs in flow graph layout code.  In some cases the
    implicit control transfers weren't honored.  In other cases,
    JUMP_FORWARD instructions jumped backwards.

    Remove unused arg from nextBlock().

pycodegen.py

    Add optional force kwarg to set_lineno() that will emit a
    SET_LINENO even if it is the same as the previous lineno.

    Use explicit LOAD_FAST and STORE_FAST to access list comp implicit
    variables.  (The symbol table doesn't know about them.)