## Issue Addressed
NA
## Proposed Changes
Whilst hacking on something I noticed that the default implementation of `FixedVector` can violate the length constraint!
E.g., `let v: FixedVector<u8; U4> = <_>::default()` would create a fixed vector with length 0, even though it promises to *always* have length 4! This causes SSZ deserialization to fail and probably other things too.
This isn't a security risk as it can't be triggered externally, however it's a foot gun for LH devs.
## Additional Info
NA
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