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	Improvements to the byte-code generator
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1. The schemeR call in coreExprToBCOs was bogusly passing a bunch of free
   variables, when the set should always be empty. As a result, compiling
   an expression with an unbound free variable (e.g. 'x + 1', where 'x' is
   entirely unbound) succeeded, expecting 'x' to be passed on the stack,
   which of course it won't be.

   This bug only shows up if an earlier stage of the compiler goes wrong,
   but fixing turns a seg-fault into a more graceful failure.

2. Make schemeE allocate non-recursive constructors directly.

3. Lots of general tidying up.  Result is 50 lines shorter than before.