[core/completion] Support scalar COMPREPLY with compgen -F

[akinomyoga]

With the current master branch,

$ bash -c '_test() { unset -v COMPREPLY; COMPREPLY=hello; }; compgen -F _test 2>/dev/null'
hello
$ bin/osh -c '_test() { unset -v COMPREPLY; COMPREPLY=hello; }; compgen -F _test 2>/dev/null'
$

This also includes a refactoring commit da6a6c1. After that, the main change is introduced in commit 87c4e62.

[andychu]

Great thank you!

[akinomyoga]

Thanks!