RCString.opAssign

assert(RCString("foo"));
assert(!RCString(""));

auto rc = RCString("foo");
assert(rc.equal("foo"));
rc = RCString("bar1");
assert(rc.equal("bar1"));
rc = "bar2";
assert(rc.equal("bar2"));

import std.range : take;
import std.utf : byCodeUnit;
rc = "bar3".take(10).byCodeUnit;
assert(rc.equal("bar3"));