Mark std.uni.MultiArray.this as safe, pure and nothrow

It does not contain any unsafe, impure and throwable operations.
It cannot be marked as nogc because it uses `new` expression.

Related issue: I found that it implicitly requires `Types.length == sizes.length` as a pre-condition.
In the current implimentation, if `Types.length < sizes.length`, some elements of `sizes` are accidentally ignored.
Can I add a pre-condition to clarify it?