ccan/io: rewrite.

I found it difficult to use myself, particularly io_duplex().

So this removes that, as well as timers and debug (for the moment).

API changes:
1) An io_plan is passed by pointer, rather than copied on the stack.
3) All io_plans are generated using the struct io_conn.
3) tal is the allocator.
4) A new connection must be set up with a callback, so this is now the
   same as one generated from a listener.
5) io_read_partial and io_write_partial take an explicit length.
6) io_always() and io_wait() take an explit in/out arg.
7) io_break() does not return a plan.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>