Add the plus operator (+) to make eval expressions simpler

Discussion started in this thread. The larger consensus was on replacing concat(a, b) with a + b (or +(a, b)?). This would reduce the number of paranthese nesting levels (at least in the case of the infix notation), thus reducing eye strain 👀 and eyeglass numbers 👓.

NOTE: The idea is to add an alias, so both concat(a, b) and a + b would work. We need to keep concat for backward compatibility and we want + for ease of use.