[DF] Eagerly declare jitted lambdas
[DF] Eagerly declare jitted lambdas

Before this patch RDF would first, eagerly, declare a dummy lambda
that contained the user-defined expression to verify that cling
could understand it; then, lazily, the actual lambda variable used
by the nodes of the computation graph was jitted, together with all
other declarations required by RDF, just before the event loop.

With this patch, the first declaration serves both purposes: it checks
that the expression is just-in-time compilable _and_ jits precisely the
definition that will be required during the event loop. This removes
redundant logic and results in strictly less work required of the
interpreter.

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