Fix remaining 'constructor' field name use

One use remained in the interactive module.
Load genes module in JS debugging UI

Only the old module name was loaded, causing the debugging interface to
fail to run. This commit corrects that behaviour.
Add true range construction

The numeric .. operator now produces a true (lightweight) range object,
which is iterable but does not precreate a list of all the values.
Ranges have their own .. operator that sets the step to use. This commit
adds these range objects to both backends.