added option quickcheck_locale to allow different behaviours for handling locales in Quickcheck; added examples of quickcheck_locale option; trying to speed up Quickcheck_Examples;

stop node execution at first unassigned exec;

discontinued Document.update_perspective side-entry (cf. 546adfa8a6fc) -- NB: re-assignment is always necessary due to non-monotonic cancel_execution;

more scalable execute/update: avoid redundant traversal of invisible/finished nodes; tuned;

less ambitious memo_eval, since memo_result is still not robust here;

less aggressive discontinue_execution before document update, to avoid unstable execs that need to be re-assigned;