Support varlena fields with single-byte headers and unaligned storage.

This commit breaks any code that assumes that the mere act of forming a tuple
(without writing it to disk) does not "toast" any fields.  While all available
regression tests pass, I'm not totally sure that we've fixed every nook and
cranny, especially in contrib.

Greg Stark with some help from Tom Lane

src/test/regress/sql/strings.sql

@@ -329,6 +329,19 @@ SELECT substr(f1, 99995, 10) from toasttest;
 
 DROP TABLE toasttest;
 
+-- test internally compressing datums
+
+-- this tests compressing a datum to a very small size which exercises a
+-- corner case in packed-varlena handling: even though small, the compressed
+-- datum must be given a 4-byte header because there are no bits to indicate
+-- compression in a 1-byte header
+
+CREATE TABLE toasttest (c char(4096));
+INSERT INTO toasttest VALUES('x');
+SELECT length(c), c::text FROM toasttest;
+SELECT c FROM toasttest;
+DROP TABLE toasttest;
+
 --
 -- test length
 --