Support SQLite

[dzikoysk]

SQLite is relatively bad solution for a database that will be updated in the future due to:

• Lack of quite a lot of crucial types
• Enums are not supported
• RETYPE operation would be limited to a given group of type affinities: https://www.sqlite.org/datatype3.html
• Constraints update require redeclaration of the whole table & full copy of its content
  • Currently, Sqiffy uses a workaround with a modification of table scheme stored in sqlite_master. (experimental)
• Inserts does not return autogenerated keys, so it requires additional select in the transaction to properly obtain last inserted row record

Todo:

• Provide a strict mode that forbids to use features that are not directly supported by this db
  • Enums
  • Constraint updates
  • Column type changes

[dzikoysk]

Experimental SQLite support should be available in 1.0.0-alpha.25. SQLite does not support schema updates directly, so I've backported them through changes to sqlite_master - the stability of this solution is unknown.