Bugs fixed

• Restore backtrace support in the PPX (#554, #556, Gabe Levi).
• Serious logic error that could cause Lwt to hang or crash (#549, reported @koen-struyve).
• All Lwt_list functions are now tail-recursive (#538, Joseph Thomas).

Additions

• Support ;%lwt syntax in the PPX (#307, Hezekiah Carty).
• Lwt_stream.iter_n (#312, Hezekiah Carty).

Miscellaneous

• Testing improvements (#536, #541, @cedlemo).
• Documentation improvements (#544, #546, #547, #553, #559, Daniil Baturin, Jason Evans, Jess Smith, Milo Turner).