Ring buffer

Implement a fixed-capacity ring buffer and process ops: ['write', x] adds x to the back if there is room and returns True, else returns False; ['read'] removes and returns the front value, or -1 if empty; ['overwrite', x] adds x to the back, evicting the oldest element if the buffer is full, and always returns True. Return the list of results for write/read/overwrite ops, in order. capacity >= 1.