Fix function parameter initialization.

Context management for parameters of functions was incorrect. To fix it we store
a context identifier which allows to identify precisely parameters location (in
memory).

Thanks to this fix, the following code produces the correct output (as it was
the case before context introduction)
``
let f: R -> R, x -> x+3;
let g: R -> R, x -> 2*f(x);

R x = g(2);
``