Bitwise updater apis

Introduced in #82 - the updater generators can now generate AddRecordFieldMask and DropRecordFieldMask methods to the record stores for fields with the bitmask directive set:

package models

type User struct {
  // ...
  SettingsMask uint8  `marlow:"column=settings&bitmask"`
}

type UserStore interface {
  // ...
  DropUserSettingsMask(uint8, *UserBlueprint) (int64, error)
  AddUserSettingsMask(uint8, *UserBlueprint) (int64, error)
  // ...
}