Allow complex joins in query expressions and support those in the in-memory driver.

There are also a few renamed symbols in this commit and column types may only have a few primitive types now.
Rename cmp!Table to var!Table.
Improve unit tests and documentation + flatten nested con-/disjunction expressions.

Multiple expressions joined with "&" or "|" are now represented as a single ConjunctionExpr or DisjunctionExpr with many sub expressions instead of multiple nested binary expressions.