WithFunc opens bug possibilities

[jamesrom]

Regarding the WithFunc, it was added just for more flexibility should a field value depends on context or anything else.

YAGNI. Can you give a specific example of a problem it solves? I can give an example of a problem it introduces: you've now got a way to get into an infinite loop without really realising it.

fields.WithFunc("foo", func(ctx context.Context) interface{} {
    return util.getValueOfFoo(ctx)
}).Onto(ctx)

// somewhere else, some utility package etc
func getValueOfFoo(ctx context.Context) string {
    log.From(ctx).Info("getting value of foo") // infinite recursion here
    return "bar"
}

Originally posted by @jamesrom in #18 (comment)

[anzdaddy]

It was just a nice-to-have. Let's remove it.