use Array.from to expand array splats

Prior to this commit, [].slice was used to turn array-like things into
arrays when needed for splats in array literals. This commit uses
Array.from instead, when available at runtime, and falls back to slice
when not. This allows a wider range of iterables to be used as splats,
on the platforms that support them (platforms missing Array.from will
also be missing more exotic iterables).

Fix gkz#963.