Add In() and IsNotIn() to Comparable, String, and Enum validators.

[DreadLordMikey]

In(params T[])

This type of validator would accept a sequence of values (the sequence). The implementation would scan the sequence and throw an exception if any element in the sequence evaluates to the value represented by the validator.

NotIn(params T[])

This type of validator would accept a sequence of values (the sequence). The implementation would scan the sequence and throw an exception if no element in the sequence evaluates to the value represented by the validator.

Justification:

This eliminates the need for verbose checks such as x == 1 || x == 3 || x == 17 || x == 21 and reduces them to x.In(1, 3, 17, 21), while x.NotIn(1, 3, 17, 21) is the natural reduction of x != 1 && x != 3 && x != 17 && x != 21.

This type of validation is appropriate for all comparable types (string, and all the numeric data types) and all Enum types.