[Rework] Convert heap to fully intrusive kvec-based implementation

[Rework] Convert heap to fully intrusive kvec-based implementation

Convert the heap implementation from pointer-based to fully intrusive
design where elements are stored directly in the kvec array.

Key changes:
- Remove heap.c, convert to macro-only header implementation
- Store elements by value in kvec_t(elt_type) instead of kvec_t(elt_type *)
- Improve cache locality by eliminating pointer indirection
- Fix swim/sink operations to properly track elements during swaps
- Update rspamd_heap_pop to return pointer to popped element
- Update memory pool destructor heap to use new intrusive API
- Update heap tests for value-based element storage

Performance benefits:
- Better cache locality (elements stored contiguously)
- No per-element allocation overhead
- Reduced memory usage (no pointer array)