Store line count in the rope nodes too.
There are some odd things going on that are not caught by the
current test suite, so I'll need to figure out what exactly is broken
and how to test for it.

                               Naive                  Ropeish
BenchmarkBufferInsertBegin  267311    ns/op      6817 ns/op  ~= 39x
BenchmarkBufferInsertEnd       113    ns/op     11108 ns/op  ~= 0.01x
BenchmarkBufferInsertMid    128285    ns/op     11969 ns/op  ~= 11x
BenchmarkBufferInsertRand   126878    ns/op     11920 ns/op  ~= 11x
BenchmarkLine                    5.85 ns/        2197 ns/op  ~= 0x
BenchmarkRowCol             919991    ns/op      1968 ns/op  ~= 467x
BenchmarkSubstr                136    ns/op       315 ns/op  ~= 0.43x
BenchmarkSubstrR                24.3  ns/o        205 ns/op  ~= 0.12x
BenchmarkTextPoint           76533    ns/op      1929 ns/op  ~= 40x
BenchmarkWord                 5071    ns/op      7418 ns/op  ~= 0.68x