Various race and crash fixes.
Naive rebalancing node implementation + "merge" size tweak.

                                Old            New        Change
BenchmarkBufferInsertBegin  17504 ns/op      5648 ns/op   ~= 3.1x
BenchmarkBufferInsertEnd    10097 ns/op      7100 ns/op   ~= 1.4x
BenchmarkBufferInsertMid    17736 ns/op      8864 ns/op   ~= 2.0x
BenchmarkBufferInsertRand   28722 ns/op     11116 ns/op   ~= 2.6x
BenchmarkLine                7543 ns/op      8083 ns/op   ~= 0.9x
BenchmarkRowCol              7760 ns/op      1431 ns/op   ~= 5.4x
BenchmarkSubstr               988 ns/op      1076 ns/op   ~= 0.9x
BenchmarkSubstrR              876 ns/op       933 ns/op   ~= 0.9x
BenchmarkTextPoint           7826 ns/op      1798 ns/op   ~= 4.3x
BenchmarkWord               15984 ns/op     16280 ns/op   ~= 1.0x