Empty group following yield causes ClassCastException

[julianhyde]

If a from expression has a yield clause followed by an empty group, Morel throws ClassCastException.

Here is an example:

- from e in [1,2,3] yield e group;
java.lang.ClassCastException: net.hydromatic.morel.ast.Core$Id cannot be cast to net.hydromatic.morel.ast.Core$Tuple
	at net.hydromatic.morel.compile.Compiler.createRowSinkFactory(Compiler.java:340)
	at net.hydromatic.morel.compile.Compiler.createRowSinkFactory(Compiler.java:317)
	at net.hydromatic.morel.compile.Compiler.compileFrom(Compiler.java:293)
	at net.hydromatic.morel.compile.Compiler.compile(Compiler.java:240)
	at net.hydromatic.morel.compile.Compiler.compileArg(Compiler.java:164)
	at net.hydromatic.morel.compile.Compiler.lambda$compileValDecl$20(Compiler.java:619)
	at net.hydromatic.morel.ast.Core$NonRecValDecl.forEachBinding(Core.java:707)
	at net.hydromatic.morel.compile.Compiler.compileValDecl(Compiler.java:614)
	at net.hydromatic.morel.compile.Compiler.compileDecl(Compiler.java:501)
	at net.hydromatic.morel.compile.Compiler.compileStatement(Compiler.java:88)
	at net.hydromatic.morel.compile.Compiles.prepareDecl(Compiles.java:116)
	at net.hydromatic.morel.compile.Compiles.prepareStatement(Compiles.java:69)
	at net.hydromatic.morel.Shell$SubShell.extracted(Shell.java:480)
	at net.hydromatic.morel.Shell.run(Shell.java:264)
	at net.hydromatic.morel.Shell.main(Shell.java:88)

A similar query without yield is OK:

- from e in [1,2,3] group;
val it = [()] : unit list

Another query with the problem:

- from e in scott.emp where e.job = "CLERK" yield e.deptno group ;
java.lang.ClassCastException: net.hydromatic.morel.ast.Core$Apply cannot be cast to net.hydromatic.morel.ast.Core$Tuple
	at net.hydromatic.morel.compile.Compiler.createRowSinkFactory(Compiler.java:340)

[julianhyde]

This is related to #32. If a step yields a single anonymous field (not a record) how is the subsequent step to refer to it?