ui.horizontal_separator: use module member 'width'
ui.width: fall back to $COLUMNS or 80 if width is None
optimized flowdiff.look_for_fix_diff

The amount of comparisons got reduced by failing as soon as possible and
avoiding the use of set() and its generator expression parameter.
look_for_fix_diff: replaced genexpr with imap + attrgetter
flowdiff: precalculate entry data byte tuples to reduce ord() usage
flowdiff: precalculate enum_izip_entries_bytes
flow: added optional decode_func parameter
flowdiff: made decode_func available through flowdiff -d