make amb-collect reliable

[vyzo]

This would sometimes fail, because the required clause is be outside the collect scope:

(begin-amb-random (let (x (amb 0 1 2)) (required (not (= 0 x))) (all-of x)))

This fixes the issue by introducing a parameter that prescribes the behaviour of amb-fail; when set, it is a thunk that continues at the end of the collection; otherwise it just raises.

Note that we have completely deviated from the original chicken egg that served as inspiration at this point; there, all-off is a scope construct (not really, but almost because it uses mutation of global variables and it's also completely undocumented) that collects from ambs within the expression context.
In the Gerbil implementation, all-of just appears within an amb scope to perform a collect+cut.

cc @eraserhd