arbitrary: add fill_slice method for AsMut<[T: Arbitrary]>

[cyphar]

This allows users to fairly easily migrate from CoreRng::fill_byte() and similar constructions without requiring them to fuss around with a new SeedableRng (or copy-paste this boilerplate) when their slice is just full of Arbitrary-able types.

Fixes #291 Signed-off-by: Aleksa Sarai [email protected]