Added function validation

[jusexton]

Fixes #50

Proposed Changes

Allow function calls to be validated like property values.

New Abstractions

Existing code is coupled to the idea of only validating property values obtained from KProperty1. A new abstract class ValueValidator was added to decouple the Validator from where the value is obtained. The ValueValidator does everything the Property class does except implementors of the class must define how a value is extracted from the given instance.

Two implementations of the class were added:

• Property which is responsible for extracting properties to be validated
• Function which is responsible for invoking methods validating their return value

Also, additional DSL factory methods were added to the validator context to allow the creation of the new Function ValueValidator

Missing Functionality

Currently functions can not be validated with given arguments. This seems like it could be addressed but for some reason I was not able to get it working.

Minimal Testing

Very little testing was done. If this approach is appreciated, I don't mind adding additional test cases.

Examples

data class Employee(val email: String? = null) {
    init {
        validate(this) {
            validate(Employee::hasEmail).isTrue()
            validate(Employee::email).isEmail()
        }
    }

    fun hasEmail() = email != null
}