Re-order pg_attribute columns to eliminate some padding space.

 Re-order pg_attribute columns to eliminate some padding space.
 
 Now that attcompression is just a char, there's a lot of wasted
 padding space after it.  Move it into the group of char-wide
 columns to save a net of 4 bytes per pg_attribute entry.  While
 we're at it, swap the order of attstorage and attalign to make for
 a more logical grouping of these columns.
 
 Also re-order actions in related code to match the new field ordering.
 
 This patch also fixes one outright bug: equalTupleDescs() failed to
 compare attcompression.  That could, for example, cause relcache
 reload to fail to adopt a new value following a change.
 
 Michael Paquier and Tom Lane, per a gripe from Andres Freund.
 
 Discussion: https://postgr.es/m/20210517204803.iyk5wwvwgtjcmc5w@alap3.anarazel.de