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	Fix two External-Core bugs
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1.  An inadvertent "let x = ...x..." bug in TcRnDriver

2.  Adjust the new -main-is story, so that the root module is called
	":Main" instead of "$Main".
    This means that the z-encoded module name is "ZCMain" rather than "zdMain",
    which in keeps the External-Core lexer happy.  And is more consistent generally.

3.  Make the renamer happy to see definitions from modules other than the "home" one,
    when doing External Core.  In the main module, there'll be a definition for
    ZCMain.main.