[RF] Minor improvements to RooFit evaluation code generation

  * fix invalid memory access in RooFuncWrapper by getting observables
    and parameters of the original function, and not the one that is
    compiled for a normalization set, which is temporary.
    What matters are the names of the parameters and observables, and
    they are the same anyway

  * new `CodeSquashContext::buildCall()` function to avoid code
    duplication

  * add support for `RooExponential`, including unit test