Many, many changes to support type-safe joins.
Real record projection.
Fixed projection tests that had ben wrongfully passing this whole time.
Fixed projector to make newly-failing tests pass.
persistence: Clear context before checking lifetime sentinel.
persistence: belongs_to access
persistence: Fixes to Projection, moved TypeList into separate header.
persistence::relational_algebra::Projection: move assignment
persistence: RecordPtr comparison.
Fixed join projection test.
persistence: Projection can now name typed joins.
persistence: Use AS-syntax in joins and selects.
removed debug print
Allow self-joins.
(failing) tests for self-joins.
Type-safe conditions for joins.
persistence: Support for arbitrary levels of self-joins.
persistence: fixed Projection::order
persistence: Projection::all
Updated blog example with newest persistence API.