[WIP] Array utilities

[akinomyoga]

I decided to move code stubs for array utilities in #739 and #950 to here.

I guess this could end up as an invalid regex if _parse_help lets any strings through to main_cmd that make it so. The previous implementation had the same issue though, so not a regression. Leaving up to you to decide whether to address or merge as is.

Some array utilities would be nice to help with cases like this. I have this ages old unfinished patch for the main bash_completion in my stash, probably broken in the first place and uses namerefs so requires bash >= 4.3, but to illustrate what I was thinking back $then

+_bashcomp_uniq()
[...]
+_bashcomp_last_index() {
[...]
+_bashcomp_compact() {
[...]
+_bashcomp_index_of() {
[...]

Originally posted by @scop in #950 (comment)

• bc6f87d This implements a function _comp_array_filter. I'm not sure whether Ville would finally find it reasonable to include the function in bash-completion, but I tried to make the interface minimal (one function _comp_array_filter) yet make the function versatile as @calestyo would be likely to request. In any way, this function can be used for the array manipulations of general cases.

Originally posted by @akinomyoga in #739 (comment)

[akinomyoga]

Ref #432