pgsql: Mostly-cosmetic improvements in memory chunk header alignmentco

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f65d21b258085bdc8ef2cc282ab1ff12da9c595c

Modified Files
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src/backend/utils/mmgr/aset.c       | 25 ++++++++++++----
src/backend/utils/mmgr/generation.c | 58 +++++++++++++++++--------------------
src/backend/utils/mmgr/slab.c       | 21 ++++++++++----
3 files changed, 62 insertions(+), 42 deletions(-)