Use SIMD for UTF-8 to UTF-32 conversion

[christianparpart]

And maybe also u8-to-u16.

Get inspired by https://github.com/BobSteagall/utf_utils/blob/master/src/utf_utils.cpp#L1126 to see how he has been doing that.

Non-supported platforms should automatically fall back to the classical algorithm.

Then test if (and if: then how much) the performance gain will be on contour throughout benchmarks.

[data-man]

Just for a record: benchmarks are in my ToDos (using very nice https://github.com/martinus/nanobench).