Introduce `free_slots` in `StackAllocator` to keep track of physical
slots that have been freed. When mapping a new virtual slot, the
allocator first attempts to reuse a slot from the `free_slots` vector
before allocating a new one.
This change also includes a new function `collect_scope_locals` that
identifies non-captured local variables defined within a block. After
these expressions are lowered, the allocator reclaims the physical slots
associated with these locals, making them available for reuse by
subsequent blocks.
A new test `test_slot_reuse_non_overlapping_scopes` is added to verify
that distinct scopes correctly reuse stack slots, ensuring optimal stack
usage.
Brummel
0088a644eb