Support variable / term at guard conditions

Guards supports only literals.  

```
h(a,b,c,x,y) :- f(a,b,c),f(x,b,y), a != 1.
```

```
h(a,b,c,x,y) :- f(a,b,c),f(x,b,y), a != x.

** exception error: {badmap,{error,{1,datalog_yeec,
                                    ["syntax error before: ",["x"]]}}}
```
