Offer ?default arguments, instead of ~labelled ones

[djs55]

Some of the functions take a metric ton of arguments, most of which have sensible defaults, which someone writing a simple client might not know for sure. The client writer can probably guess, but they shouldn't have to and it makes the code quite unwieldy.

Where the protocol requires an argument, we should generate

let f ?(x = Reasonable_default)

Where the protocol allows an argument to be omitted, we should generate

let f ?x

[gaborigloi]

It might be safer to just make the code break in case we add a new parameter, so that clients will be notified about it, instead of just silently sending the default value.