[MLton] symbol visibility

[Matthew Fluet]

Commit r6680 adds and commit r6785 modifies the CFunction.SymbolScope.t 
datatype.  When emitting a c-prototype, the symbol scope is translated 
thus:
       val symbolScope =
          case symbolScope of
             SymbolScope.External => "IMPORTED "
           | SymbolScope.Private => "INTERNAL "
           | SymbolScope.Public => "EXPORTED "
which is then expanded by cpp via the "export.h" header.

Is there a good reason to have a different names (i.e., External for 
SymbolScope.t, but "IMPORTED" for the macro)?