Internal improvements

[Fs02]

• Cache some functions that use reflection.
• Ability to avoid reflection on some cases by implementing inferfaces:

// Defines fields in a struct, used mainly in changeset.
type fields interface {
	Fields() map[string]int
}

// Defines type of fields in a struct, used mainly in changeset.
type types interface {
	fields
	Types() []reflect.Type
}

// Defines values of fields in a struct, used mainly in changeset.
type values interface {
	fields
	Values() []interface{}
}

// Defines database scanners, used when scanning result from database rows.
type scanners interface {
	Scanners([]string) []interface{}
}