std.unicode.utf8ToUtf16Le: improve performance

on a simple test input:

original utf8ToUtf16Le: elapsed: 111384483 ns (111 ms)
new utf8ToUtf16Le: elapsed: 138570 ns (0 ms)

it's 800x faster in debug mode and ~4500x faster in release-fast mode.

this was slowing down installation of files on windows in build scripts.
c3d20373eeAndrew Kelley committed on 7/15/2019, 11:35:10 PMparent851a728
2 files changedLine totals unavailable