feat(validate): initial implementation

This is a bare-bones implementation of a basic validation package based
around a very simply Validatable interface:

    type Validatable interface {
        Validate() error
    }

The goal is to keep things as simple as possible, while also giving as
much control as possible over how validation logic is performed.

helpers.go

@@ -0,0 +1,36 @@
+package validate
+
+import (
+	"reflect"
+)
+
+// RequireField returns a Error type for the given field if provided value is
+// empty/zero.
+func RequireField(field string, value interface{}) error {
+	err := &Error{Field: field, Msg: "is required"}
+	v := reflect.ValueOf(value)
+
+	if v.Kind() == reflect.Ptr {
+		if v.IsNil() {
+			return err
+		}
+		v = v.Elem()
+	}
+
+	if !v.IsValid() {
+		return err
+	}
+
+	switch v.Kind() { //nolint:exhaustive
+	case reflect.Map, reflect.Slice:
+		if v.Len() == 0 {
+			return err
+		}
+	default:
+		if v.IsZero() {
+			return err
+		}
+	}
+
+	return nil
+}