engine: implement engine

[tpdenk]

Implement an engine that can execute our IR.

Scope:

• selection support ( engine: extend selection support #14 )
  • fix literal evaluation ( engine: cleanly resolve literals to values from intermediate table #13 )
• define file format ( engine: define file format #3 )
• file system access ( engine: file system access #16 )
• full table scan ( engine: full table scan #17 )
• e2e tests ( engine: example database test setup #18 )
• db configuration ( engine: database-specific configuration #20 )
• support for PCTFREE and overflow pages ( engine: PCTFREE and overflow pages #19 )
• introduce transaction manager ( engine: transaction manager #21 )
• profiling ( engine: fine profiling #23 )
• builtins ( engine: implement builtin functions #2 )
• add documentation documents (xdbdoc)