Rewrite format_number_range

[tautschnig]

The previous code contained an unreachable instruction "next=true;" As I could not figure out where it belongs I ended up rewriting it, now using a lot less nesting and putting all the state into the iterator and a single start_number variable instead of several Booleans.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 70d1a85).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85741077