Do not use `typeof(<type>[0])` to get the element type of an array.

Use `typeof(<type>.init[0])` instead as Issue 6408 is fixed.

Issue 6408 URL: http://d.puremagic.com/issues/show_bug.cgi?id=6408
Do not slice types as it is prohibited now.

Probably the reason is the same as for the previous commit.

Changed `unstd.multidimensionalarray.R` implementation.