GHSA-gfgm-chr3-x6px: prettytable-rs: Force cast a &Vec<T> to &[T] may lead to undefined behavior

In function Table::as_ref, a reference of vector is force cast to slice. There are multiple problems here:

1. To guarantee the size is correct, we have to first do Vec::shrink_to_fit. The function requires a mutable reference, so we have to force cast from immutable to mutable, which is undefined behavior (UB).
2. Even if (1) is sound, &Vec<T> and &[T] still might not have the same layout. Treating them equally may lead to undefinted behavior (UB).