refactor: reformat and add LDocs to astr.lua.

* lib/zile/astr.lua: Reformat to match other object modules.
Add LDocs.
* doc/config.ld (file): Add lib/zile/astr.lua.
* local.mk (dist_classes_DATA): Add lib/zile/astr.lua.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>