Introduce a table into a query without using the Beamable and Database machinery.
The first argument is the optional name of the schema the table is in and the second is the name
of the table to source from.
The third argument is a tuple (or any nesting of tuples) where each value is of type NamedField
(use field_ to construct).
The return value is a tuple (or any nesting of tuples) of the same shape as structure but where
each value is a QExpr.
For example, to source from the table Table1, with fields Field1 (A boolean), Field2 (a
timestamp), and Field3 (a string)
table_ Nothing "Table1" ( field_ @Bool "Field1", field_ @UTCTime "Field2", field_ @Text "Field3" )