Remove min payload size restriction from the compiler and RTS

Previously we'd require every heap object to have at least one word space in the payload. This was incorrectly documented as follows in the source code: (now deleted)

Minimum closure sizes

This is the minimum number of words in the payload of a
heap-allocated closure, so that the closure has enough room to be
overwritten with a forwarding pointer during garbage collection.

This comment was wrong: the GC doesn't require a location in the payload, forwarding pointers are made using the info table pointer location.

The real reason why we needed a word in the payload is because mark-compact collector allocates one bit per word in the heap for bitmap, but needs two bits per object to work efficiently. Details are explained in the new note Note [Mark bits in mark-compact collector] in Compact.h.

We now allocate two bits in the bitmap per word and remove the restriction. Heap objects can now have size 1, as demonstrated in the updated tests.

Nofib results:

• 0.0% change in binary sizes and TotalMem
• "contaraints" now allocates -0.1% less. No other changes in allocations.
• Runtime/Elapsed results are noisy and inconclusive