Concat operator, tests. Now [1,2,3] + (4..6) works!

Fourth try. ;)

Concat operator, tests. Now [1,2,3] + (4..6) works! Inlined helper functions to keep core lib small.

I could not get `this[0..-1]` to work for copying, so I inlined the method I had before.

Can develop be kept updated to master? When I added a feature branch (automatically, so my fault), it inadvertently was based on develop.