lib/helpers: fix all_groups()

- Don't write to disk, just pipe.
- Don't loop, just do all functions.

Performance of old implementation on my system:
```
real    0m9.996s
user    0m5.318s
sys     0m9.126s
```

Performance of new implementation on my system:
```
real    0m0.052s
user    0m0.069s
sys     0m0.025s
```

lib/helpers.bash

@@ -811,13 +811,7 @@ function all_groups() {
 	about 'displays all unique metadata groups'
 	group 'lib'
 
-	local func file
-	file=$(mktemp -t composure.XXXX)
-	for func in $(_typeset_functions); do
-		declare -f "$func" | metafor group >> "$file"
-	done
-	sort -u "$file"
-	rm "$file"
+	declare -f | metafor group | sort -u
 }
 
 if ! _command_exists pathmunge; then