Implement Zeroize for [MaybeUninit<Z>]

[elichai]

Implementing Zeroize for [MaybeUninit<Z>] is very helpful IMHO, because it allows to zeroize any custom heap allocated objects by just constructing a slice of MaybeUninit(as some of the buffer might not be initialized) and calling zeroize() on it.

I also changed the Vec implementation to zero out the full capacity and not just the uninitialized part, because I don't see any difference between the initialized and the unintialized, if the data in the buffer is secret then we should make sure we zero out all of the buffer.

The downside is that if the Vec contains a primitive(say Vec<u8>) then we'll zero the used space twice, so I can see arguments against it (currently the only time I can think this will matter is in Vecs with another indirection, like Vec<Box<Z>> or Vec<&Z>, and then is there a point in zeroing out those pointers?)

So if people disagree on the 2nd commit I can drop it and leave only the first commit.

[codecov-commenter]

Codecov Report

Merging #793 (a939587) into main (86b87a3) will increase coverage by 0.14%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##             main     #793      +/-   ##
==========================================
+ Coverage   55.43%   55.58%   +0.14%     
==========================================
  Files          76       76              
  Lines        2785     2794       +9     
==========================================
+ Hits         1544     1553       +9     
  Misses       1241     1241              

Impacted Files	Coverage Δ
zeroize/src/lib.rs	84.04% <86.66%> (+1.68%)	⬆️

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[elichai]

Fixed the CI fail

[tony-iqlusion]

Thank you!