Add cut_characters implementation, based on cut_bytes

This implementation uses rust's concept of characters and fails if the
input isn't valid utf-8. GNU cut implements '--characters' as an alias
for '--bytes' and thus has different semantics, for this option, from
this implemtation.

Makefile

@@ -13,6 +13,7 @@ PROGS       := \
   cksum \
   comm \
   cp \
+  cut \
   dirname \
   echo \
   env \