`slice_of` internal iterators get invalidated on move

[ashtum]

Even a simple case like buffers::slice_of<buffers::const_buffer_pair> breaks on move, so we can't keep it as a member variable inside composed operations.

buffers/include/boost/buffers/slice.hpp

Lines 67 to 73 in 908fc2e

	BufferSequence bs_;
	iter_type begin_;
	iter_type end_;
	std::size_t len_ = 0; // length of bs_
	std::size_t size_ = 0; // total bytes
	std::size_t prefix_ = 0; // used prefix bytes
	std::size_t suffix_ = 0; // used suffix bytes

[MungoG]

Per the discussion in slack, this should be resolved by storing index values rather than iterators.