Allow cursor to be moved forwards

[OFFTKP]

As mentioned in #13, allows for backpatching instructions then moving back to the previous position.

Example usage:

ptrdiff_t cursor = as.GetCodeBuffer().GetCursorOffset();
as.RewindBuffer(jump.location);
Emitter::EmitJump(*this, &block_map[jump.target]);
as.AdvanceBuffer(cursor);

Asserts changed in GetOffsetPointer to allow for emitting branches to Labels that point forwards in the buffer.

Feel free to suggest changes if this implementation is not optimal.

[lioncash]

Works for me, thanks for the patch!