Simplify tagging and ref encoding.

Every "reference" is now a Path, i.e. a List[Symbol].
Create a TableExpansion when creating a TableNode.

This way the expandTables phase is no longer needed. We can also remove
the expandOn function from TableNode and use the initial TableExpansion
everywhere.

TableNode and TableExpansion are still separate Nodes so we can remove
TableExpansions after flattening all projections.
Find all referenced table columns in inferTypes.

After the introduction of nominal types for Bind nodes, the inferTypes
phase no longer detected references into tables that got aliased by
such a nominal type. This went unnoticed because the later compiler
phases do not use these types at the moment.
Implement the new column expansion and flattening scheme.

This is a complete rewrite and redesign of the central phases of the
query compiler. The new drill is:

- expandTables: The old algorithm expanded tables into their star
  projection at the place where a table was introduced, and then
  propagated the necessary values outwards. But a table's star
  projection is only relevant for the client side, so there is only one
  place where we expand it in the new scheme: At the outermost layer of
  a query in its return value. Since we know the query's result type, we
  can simply take all the NominalTypes with TableIdentitySymbols in that
  type, find the matching TableExpansions in the tree and wrap the whole
  tree in a mapping operation that copies the return value (purely based
  on its type), expanding the tables in it.

- expandRecords: Expand paths to ProductNodes and StructNodes into
  ProductNodes and StructNodes of paths, preserving the types and the
  paths into the expanded structure.

- flattenProjections: Bring all Pure nodes into a Pure(StructNode) form
  where the StructNode children are individual columns.

- relabelUnions: Still the old implementation. It should be possible to
  write a simpler type-aware version.

- pruneFields: Remove unreferenced fields from the Pure(StructNode))
  nodes created earlier. If such a Pure's TypeSymbol is not referenced
  at all (i.e. it occurs at the outer layer below the ResultSetMapping
  or inside an aggregation function), change it to a ProductNode or an
  individual column.

The assignUniqueSymbols phase is also extended to assign unique identity
TypeSymbols for TableNode and Pure nodes in addition to replacing term
symbols.

convertToComprehensions is modified to introduce identity mappings for
all TableNodes because the old comprehension fusion algorithm relies on
this shape but the new column flattening no longer produces these
extraneous mappings.
Remove some cruft from the AST.

Now that the old column flattening phases are gone, RefNode serves no
useful purpose, and DefNode was overused, too.
Merge pull request #197 from slick/tmp/new-columnization

Implement the new column expansion and flattening scheme.