Add LLProcess::ReadPipe::find() methods, with corresponding npos.

If it's useful to have contains() to tell you whether incoming data contains a
particular substring, and if it's useful for contains() and peek() to accept
an offset within that data, then it's useful to allow you to get the offset of
a desired substring within that data. But of course a find() returning offset
needs something like std::string::npos for "not found"; borrow that
convention.
Support both find(const std::string&) and find(char); the latter permits a
more efficient implementation. In fact, make find(string) recognize a string
of length 1 and leverage the find(char) implementation.
Given that, reimplement contains(mumble) as shorthand for find(mumble) != npos.
Implement find() overloads using std::search() and std::find() on
boost::asio::streambuf character iterators, rather than copying to std::string
and then using string search like previous contains() implementation.
Reimplement WritePipeImpl::tick() and ReadPipeImpl::tick() to write/read
directly from/to boost::asio::streambuf data, instead of copying to/from a
temporary flat buffer.
As long as ReadPipeImpl::tick() keeps successfully filling buffers, keep
reading. Previous implementation would only handle a long child write over
successive tick() calls. Stop on read error or when we come up short.