add stream_append, stream2string

todo: test for memory leaks
sorted directory streams
move getfile and putfile to Chj/
xputfile_utf8: print nested data structures
create
create Chj::FP::url_
de-duplicate stream_*_fold_right
add stream__subarray_fold_right, stream__subarray_fold_right_reverse
add subarray2stream
add subarray2stream_reverse
create Chj::FP::OrderedCollection

But 'maybe' doesn't work for also returning streams.
make () the Perl 'maybe' way?

It will work well for if (my ($foo)= ...) { }
rename maybe_* to perhaps_* to indicate () instead of undef result
add new_from_array
add maybe_next, maybe_prev
add xcopyfile_utf8
PXML serializer: evaluate CODE
PXML serializer: evaluate CODE in attribute values, too (hack)
create Chj::PSVG
fix license statements