dump() instantiates the serializer for the whole value type, which does
not compile for object key types it cannot write (e.g., the
custom_key_test::key_c_str of #5795), even though the value is always a
number here. Produce the decimal digits directly instead; they are
generated per digit so that the minimum of a signed type is never
negated as a whole.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>