Ensure slice size constraints in uniform functions.

[emilio]

Some uniform functions asume that multiples of x elements are passed to them.

It could be worth it to add debug_assert!s to those functions to ensure they're been called correctly.

It's not such a big deal (in the sense that it doesn't cause unsafety), because of the round-down in integer division, but it's nice to have to catch bugs.

In #78 I pinged @glennw, but since there were no response, it was merged as-is.