Disabling an optimization (converting top-level letrefs to var
definitions), which was introduced to implement the return-array
optimization. The reason is because the introduction of such a letref
bound variables was causing the AutoLUT to think it was a global
in/out variable.

Now, the optimization is disabled, but code generation for functions
is smarter to take into account letrefs, not only vars.
Merge branch 'release' of github.com:dimitriv/Blink into performance