Complexity: m log(n) (where m is the number of elements to remove)
Example:
import std.algorithm, std.container;
auto rbt = redBlackTree!true(0, 1, 1, 1, 4, 5, 7);
rbt.removeKey(1, 4, 7);
assert(equal(rbt[], [0, 1, 1, 5]));
rbt.removeKey(1, 1, 0);
assert(equal(rbt[], [5]));