cpp/iterator/incrementable traits

Computes the associated difference type of the type, if any. Users may specialize for a program-defined type.

@1@ Primary template is an empty struct. @2@ Specialization for pointers. Provides a member type equal to std. @3@ Specialization for const-qualified types. @4@ Specialization for types that define a public and accessible member type. Provides a member type equal to. @5@ Specialization for types that do not define a public and accessible member type but do support subtraction. Provides a member type equal to. The implicit expression variations rule (see below) applies to the expression.