Fixup stout/stderr on Windows

Fixup stout/stderr on Windows

WriteConsoleW can fail if called with a large buffer so we need to slice
any stdout/stderr output.
However the current slicing has a few problems:
 1. It slices by byte but still expects valid UTF-8.
 2. The slicing happens even when not outputting to a console.
 3. panic! output is not sliced.

This fixes these issues by moving the slice to right before
WriteConsoleW and slicing on a char boundary.