compiler: elide nil var intializaton.

Virtual machine local variables registers don't require
nil initialization. In cases when a complex variable
initializer is known to return nil, we can elide the move
instrution which moves that nil into the register.

* share/txr/stdlib/compiler.tl (null-reg): New function.
(compiler comp-let): Don't move the value of the output
register of the init fragment into the variable, if that
register is the null register t0.

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