A Bidirectional iterator to a query parameter.
using iterator = see-below;
Objects of this type allow iteration through the parameters in the query. Any percent-escapes in returned strings are decoded first. The values returned are read-only; changes to parameters must be made through the container instead, if the container supports modification.
The strings produced when iterators are dereferenced belong to the iterator and become invalidated when that particular iterator is incremented, decremented, or destroyed.
The implementation may use temporary, recycled storage to store decoded strings. These iterators are meant to be used ephemerally. That is, for short durations such as within a function scope. Do not store iterators with static storage duration or as long-lived objects.