std: simplify utf8ToUtf16Le

Also faster, on my machine unicode/throughput_test.zig now gives e.g.
> original utf8ToUtf16Le: elapsed: 1048 ns (0 ms)
> new utf8ToUtf16Le: elapsed: 971 ns (0 ms)
ab6065407ddaurnimator committed on 12/28/2019, 5:49:59 AMparent5843a6e
1 file changedLine totals unavailable