wrire_sink design

[vinniefalco]

The current requirements for WriteSink are

struct write_sink
{
  void size_hint( std::size_t );

  template< class Buffers >
  system::error_code write( Buffers const&, bool finished );

  system::error_code finish();
};

There is some friction here. First, size_hint is not called when the size is unknown, which is a pessimization. It would be better for this function call to be required and explicit:

void size_hint( optional<std::size_t> );

This way write doesn't need stateful logic to know that size_hint wasn't called in the case where the size is unknown.

Also, the buffer sequence requirement implies that write is always a template. Or that we need a perfect type-erasing wrapper. Neither of these are particularly satisfying.