Change Vector<T> to use more compact code using S.R.CS.Unsafe (#26637)

S.R.CS.Unsafe is used in number of places in Vector<T> already. This change modifies more places to use it to make the code more compact.

Updated Vector.tt template to match while I was on it

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

Author: jkotas