improve converting to Go string performance

[ffmiruz]

Due to out of place performance on this bench for large case i thought there could be easy performance wins.
Profiling on that, performance hit as the string got bigger due to this loop https://github.com/zombiezen/go-sqlite/blob/f46dbf8b767e074a46642cd87c3d0e0cafcc79b2/sqlite.go#L1297C2-L1308

This PR instead convert to unsafe string all at once and then clone it to get safe string. The performance for 1 byte string is the same and then keep improving as the string get bigger.
~40% improvement for the large case on the benchmark. Also improve performance of other benchmark cases slightly.

[zombiezen]

Thank you so much for getting to the bottom of this! I shuffled the code around slightly so that it's converting from []byte to string to avoid the strings.Clone call, which seems a little subtle to me.

I realized I don't have benchmarks for this library generally, so I added a small microbenchmark that proves this speedup. Went from 30ns/op to 15ns/op on my machine, nice work!