utils/types: toggle_set: TypeError on string
utils/types: toggle_set: TypeError on string

Strings are iterable, so can be used to instantiate sets (resulting in a
set of chars). This is never what we want for toggle_set's though, and
may result in difficult-to-interpret errors when parsing configuration,
so raise a TypeError if attempting to create a toggle_set with a string.

ARM-software/workload-automation,lisatn/workload-automation,ARM-software/workload-automation,setrofim/workload-automation,lisatn/workload-automation,setrofim/workload-automation,setrofim/workload-automation,lisatn/workload-automation,ARM-software/workload-automation,ARM-software/workload-automation,lisatn/workload-automation,setrofim/workload-automation