wally_psbt_to_bytes() difficult to use

Without any way of knowing the length to allocate, how do we call wally_psbt_to_bytes?

For the moment, we allocate 1k and loop, doubling each time.
But worse, it returns WALLY_EINVAL for reasons OTHER than "you didn't give enough room".

There are two ways of fixing this:
1. Expose psbt_get_length(). This is inefficient.
2. Change the semantics of bytes_written to "bytes_needed", and set it to the number of bytes you *want*. Then return WALLY_ENOMEM to distinguish from unrelated errors.

The second allows a more optimal implementation in future.