Clean up get_maximum_aligned_size

[qinsoon]

This PR fixes #730.

• Alignment offset is now usize rather than isize. We never allow negative values, and it was a mistake to use isize.
• Seperate alignment padding calculation from align_alloc and get_maximum_aligned_size, update the implementation and add unit tests.
• get_maximum_aligned_size also requires offset to work properly.
  • The mimalloc allocator (FreeListAllocator) is updated. As we would need to pass around both align and offset, we simply calculate bin once, and pass it around instead.
• Default value as known_alignment should be VM::ALLOC_END_ALIGNMENT rather than MIN_ALIGNMENT.