This is the first part of my effort to make the eth2 crate more lightweight.

Proposed Changes

Remove ZeroizeString from account_utils, which uses the Zeroize derive macro from the zeroize crate, and use the Zeroizing<T> newtype provided by zeroize instead.

Our newtype did not add any value, except providing without_newlines, which replaced a one-liner and was used in only one location (outside of tests).

Additional Info

eth2 was depending on account_utils exclusively for ZeroizeString, which was the inspiration for this PR. Removing the account_utils dependency immediately made the crate lighter: cargo tree -p eth2 | wc -l went from 1995 down to 1865.

There are more newtypes scattered in the codebase that could be removed like this, but some arguably improve code readability by providing context specific names, such as DerivedKey and LamportSecretKey, so I left them alone for now.