Introduce stack which recycles the entries

[overlookmotel]

Semantic includes a stack structure UnresolvedReferencesStack which stores a bunch of hash maps and recycles them as go up and down the stack.

Generalize this to a NonEmptyRecycleStack structure, which we can also use in many places in transformer.

Additionally, optimize it by using pointers instead of len + capacity, the same way Stack and NonEmptyStack do:

pub struct NonEmptyRecycleStack<T: Default> {
    /// Pointer to current entry on stack.
    /// Points *to* current entry, not *after* current entry.
    cursor: NonNull<T>,
    /// Pointer to start of allocation (first entry)
    start: NonNull<T>,
    /// Pointer to after last filled slot
    end_filled: NonNull<T>,
    /// Pointer to end of allocation
    end: NonNull<T>,
}