GHSA-66p5-j55p-32r9: smallvec creates uninitialized value of any type

Affected versions of this crate called mem::uninitialized() to create values of a user-supplied type T. This is unsound e.g. if T is a reference type (which must be non-null and thus may not remain uninitialized).

The flaw was corrected by avoiding the use of mem::uninitialized(), using MaybeUninit instead.