Concatenate and leading zeroes

[JCumin]

If all elements of the input list are bound, we should relax the constraint that there cannot be any leading zero.
If they are not all bound, then the behavior stays the same as before (to avoid infinite choice points).

This was brought up by @ais523 in this PPCG comment

[JCumin]

This commit solves this problem ONLY IF the input is ground and the output is a variable (which is typically when the problem occurs).

It still doesn't work properly when both the input and the output are ground. Concatenate's implementation is really messy and that's hard to implement without breaking everything.