let makes variables local to current context

[project-eutopia]

Previously using a child context derived from context.spawn_child would not define local variables when using let. This was because the local variable was not being passed on as an argument to successive calls to register_variable!. This fixes this problem.

[coveralls]

Coverage increased (+0.009%) to 97.561% when pulling d2faa91 on let_for_local_variables_in_children into b5b6477 on master.