Disallow mut ref to inner Vec for NEVec, allow ref and mut ref to slice

[MOZGIII]

Allowing mut access to inner Vec much allows breaking the non-emptyness invariant.

[MOZGIII]

@fosskers there's more work to do in this domain, like NESet also needs to be cleaned up and etc - but first I'd like your feedback on this.

[fosskers]

Can you provide a code example that demonstrates the invariant being violated?

[MOZGIII]

let mut nevec: NEVec<u8> = ...;

let vecmut: &mut Vec<u8> = nevec.as_mut();

*vecmut = vec![];

[fosskers]

Thanks, I'll test with this.