Use wrappers of PG_DETOAST_DATUM_PACKED() more.

This makes almost all core code follow the policy introduced in the
previous commit.  Specific decisions:

- Text search support functions with char* and length arguments, such as
  prsstart and lexize, may receive unaligned strings.  I doubt
  maintainers of non-core text search code will notice.

- Use plain VARDATA() on values detoasted or synthesized earlier in the
  same function.  Use VARDATA_ANY() on varlenas sourced outside the
  function, even if they happen to always have four-byte headers.  As an
  exception, retain the universal practice of using VARDATA() on return
  values of SendFunctionCall().

- Retain PG_GETARG_BYTEA_P() in pageinspect.  (Page images are too large
  for a one-byte header, so this misses no optimization.)  Sites that do
  not call get_page_from_raw() typically need the four-byte alignment.

- For now, do not change btree_gist.  Its use of four-byte headers in
  memory is partly entangled with storage of 4-byte headers inside
  GBT_VARKEY, on disk.

- For now, do not change gtrgm_consistent() or gtrgm_distance().  They
  incorporate the varlena header into a cache, and there are multiple
  credible implementation strategies to consider.

src/pl/plpython/plpy_typeio.c

@@ -603,9 +603,9 @@ PLyLong_FromOid(PLyDatumToOb *arg, Datum d)
 static PyObject *
 PLyBytes_FromBytea(PLyDatumToOb *arg, Datum d)
 {
-	text	   *txt = DatumGetByteaP(d);
-	char	   *str = VARDATA(txt);
-	size_t		size = VARSIZE(txt) - VARHDRSZ;
+	text	   *txt = DatumGetByteaPP(d);
+	char	   *str = VARDATA_ANY(txt);
+	size_t		size = VARSIZE_ANY_EXHDR(txt);
 
 	return PyBytes_FromStringAndSize(str, size);
 }