refactor TruncIndex to use a trie & vendor deps

This replaces suffixarray with a go-patricia trie.

```
10 second test BEFORE:
PASS
BenchmarkTruncIndexAdd100            500          57708465 ns/op         6109651 B/op       1414 allocs/op
BenchmarkTruncIndexAdd250             50         414062258 ns/op        36072887 B/op       3797 allocs/op
BenchmarkTruncIndexAdd500             10        1766381236 ns/op        140041404 B/op      7940 allocs/op
BenchmarkTruncIndexGet100         100000            260940 ns/op          362320 B/op        450 allocs/op
BenchmarkTruncIndexGet250          10000           1127151 ns/op         2198736 B/op       1126 allocs/op
BenchmarkTruncIndexGet500           5000           3336467 ns/op         8734328 B/op       2252 allocs/op
BenchmarkTruncIndexNew100          10000           1259865 ns/op          170950 B/op        238 allocs/op
BenchmarkTruncIndexNew250           5000           3510118 ns/op          405243 B/op        553 allocs/op
BenchmarkTruncIndexNew500           2000           7792312 ns/op          820164 B/op       1074 allocs/op
ok      docker/pkg/truncindex     186.085s



10 second test AFTER:
PASS
BenchmarkTruncIndexAdd100         100000            285569 ns/op          135162 B/op       2433 allocs/op
BenchmarkTruncIndexAdd250          20000            782660 ns/op          358482 B/op       6173 allocs/op
BenchmarkTruncIndexAdd500          10000           1666162 ns/op          719209 B/op      12413 allocs/op
BenchmarkTruncIndexGet100         200000             94354 ns/op           31506 B/op        737 allocs/op
BenchmarkTruncIndexGet250         100000            250821 ns/op           78868 B/op       1841 allocs/op
BenchmarkTruncIndexGet500          50000            524279 ns/op          155941 B/op       3684 allocs/op
BenchmarkTruncIndexNew100         100000            266988 ns/op          136154 B/op       2453 allocs/op
BenchmarkTruncIndexNew250          20000            746447 ns/op          361445 B/op       6205 allocs/op
BenchmarkTruncIndexNew500          10000           1587854 ns/op          714249 B/op      12394 allocs/op
ok     docker/pkg/truncindex     219.076s
```

Fixes #4091
I think we should cry here about ambiguous images.