Implement trimming of `compctl` completion lists

[marlonrichert]

There are still quit a few 3rd-party completion functions (mainly Python-related) that still rely on compctl. Zsh Autocomplete, however, can currently trim only completion lists generated by compadd. Thus, any completion that relies on compctl can easily result in the dreaded zsh: do you wish to see all XXX possibilities (YYY lines)? eating up your keystrokes.

Implementation notes:

• _default calls compcall to generate compctl completions.
• compctl -L <command> will print output in the form of compctl -K <function> <command>.
• <command> is stored in $_comp_command.
• <function> returns its list of completions in $reply.