Mostly-cosmetic improvements in memory chunk header alignment coding.
Add commentary about what we're doing and why. Apply the method used for
padding in GenerationChunk to AllocChunkData, replacing the rather ad-hoc
solution used in commit
7e3aa03b4. Reorder fields in GenerationChunk so
that the padding calculation will work even if sizeof(size_t) is different
from sizeof(void *) --- likely that will never happen, but we don't need
the assumption if we do it like this. Improve static assertions about
alignment.
In passing, fix a couple of oversights in the "large chunk" path in
GenerationAlloc().
Discussion: https://postgr.es/m/E1eHa4J-0006hI-Q8@gemulon.postgresql.org