Replace "expression" command with a prefix-command.

Replace "expression" command with the prefix-command "compile".  This command now
accepts two sub-commands

	compile code [-r|-raw] [CODE]
	compile file [-r|-raw] FILENAME

The sub-command "file" now completes on filename.

The sub-command "code" acts much like the old "expression" command.

Add a alias to the prefix-command "compile" called "expression".