allow full primitive expressions to be passed as function parameters

[ostap]

currently bandicoot supports only constant expressions to be passed to the functions, e.g. the following fragment works fine:

fn GetBooks(type string) Books {
    return select (genre == type) shelf;
}

# a constant expression
GetBooks "novel";

but the following does not work so far:

fn GetInexpensive(genre string) Books {
    return select (price < 10.0) (GetBooks genre);
}

more than that, the error message is confusing:

unknown identifier 'genre' neither function call nor variable declared with this name