Improve ForwardList.clear()

[samchon]

When call ForwardList.clear() method, it re-generates the iterator objects, especially the ForwardList.end() iterator. It can cause mis-iteration when the ForwardList.clear() method is called during the iteration routine.

tstl/src/container/ForwardList.ts

Lines 146 to 152 in ac68a64

	public clear(): void
	{
	this.end_ = ForwardList.Iterator.create(this.ptr_, null!, null!);
	this.before_begin_ = ForwardList.Iterator.create(this.ptr_, this.end_);
	this.size_ = 0;
	}