Precise timestamp, proper default and exported keys

• precision for Timeable. It enables microseconds for timestamp, datetime and time
• migrations table has been updated to support microseconds
• default values for expressions can be used for Enum, JSON, Text and String column types. To make an expression, use curly braces, eg. (UUID())
• Empty string can be used for default in Enum, JSON, Text and String column types. Use <empty> or <nil> as default value
• Foreign and Key became exported, so they can be used with alterTableCommand