Force cast a &Vec<T> to &[T]

In function Table::asref, 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::shrinktofit. The function requires a mutable reference, so we have to force cast from immutable to mutable, which is UB. 2. Even ...

Flaw in `realloc` allows reading unknown memory

When reallocing, if we allocate new space, we need to copy the old allocation's bytes into the new space. There are oldsize number of bytes in the old allocation, but we were accidentally copying newsize number of bytes, which could lead to copying bytes into the realloc'd space from past the chu...