Allocating objects of sizes that are not a multiple of MIN_ALIGNMENT causes assertion fail

[qinsoon]

The assertion here will check if the cursor after last allocation (region) is aligned to MIN_ALIGNMENT:

mmtk-core/src/util/alloc/allocator.rs

Line 46 in 02ca036

debug_assert!(region.is_aligned_to(MIN_ALIGNMENT));

It is ported from the Java MMTk:
https://github.com/JikesRVM/JikesRVM/blob/682344f6eb9b9c3a75b8ad54b62ceb4b133b94f5/MMTk/src/org/mmtk/utility/alloc/Allocator.java#L100

However, this assertion seems to be incorrect. If we allocate an object whose size is not a multiple of MIN_ALIGNMENT, e.g. 6 when MIN_ALIGNMENT=4. The assertion will fail for the next allocation.

Thanks for @udesou for reporting this bug.