Handle the pretty-printer import correctly

Use the meta-library and handle that oputside __echo (i.e. do not try to
import it kajillion times).

include/core.sh

 #!/bin/bash
 
+ffoo import mkpretty
+
 
 ##--------------------##
 ## PRINTING FRONT-END ##
     #   -u patt [patt...] convert each patt to usage pattern
     #   -v var [var...] show contents of each var
     #
-    ffoo import mkpretty_${FFOO_MKPRETTY}
     local frontend  # who (of core.sh) was called (=> prettyprinter choice)
     local caller    # which user's function (or script) called it
     frontend="$(__cut_stack '^__echo' | head -1 )"