[Eva] Backward propagates reduction of builtins arguments.

During the interpretation of a function call, the possible values of formal
parameters can be reduced — especially by the evaluation of the function
preconditions.
At the end of a function call, Eva backward propagates these reductions to the
concrete arguments of the call.

This commit also does this backward-propagation when a builtin is used to
interpret the call.

Also removes unused [builtin] field from call results (it was only used to
avoid the backward propagation of argument reductions when a builtin was used).