py/gc: Improve mark/sweep debug output.
Previously, mark operation weren't logged at all, while it's quite useful to see cascade of marks in case of over-marking (and in other cases too). Previously, sweep was logged for each block of object in memory, but that doesn't make much sense and just lead to longer output, harder to parse by a human. Instead, log sweep only once per object. This is similar to other memory manager operations, e.g. an object is allocated, then freed. Or object is allocated, then marked, otherwise swept (one log entry per operation, with the same memory address in each case).