Improve type handling of varlena structures

Improve type handling of varlena structures

This commit changes the definition of varlena to a typedef, so as it
becomes possible to remove "struct" markers from various declarations in
the code base.  Historically, "struct" markers are not the project style
for variable declarations, so this update simplifies the code and makes
it more consistent across the board.

This change has an impact on the following structures, simplifying
declarations using them:
- varlena
- varatt_indirect
- varatt_external

This cleanup has come up in a different path set that played with
TOAST and varatt.h, independently worth doing on its own.

Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aW8xvVbovdhyI4yo@paquier.xyz