Speed up pattern matching for large lists

[poke1024]

This actually has no effect in its current form (i.e. in master).

When applied to the qtime branch (#619) though, it speeds up things quite considerably and effectively gives Mathics a whole new, snappy feeling when working with large lists.

Before (i.e. untampered #619):

In[1]:= x = Range[100000]; Timing[Combinatorica`BinarySearch[x, 100]]
Out[1]= {0.142646, 100}

After:

In[1]:= x = Range[100000]; Timing[Combinatorica`BinarySearch[x, 100]]
Out[1]= {0.000985, 100}

In the non-ordered case, leaf_candidates is actually only used in the call to subranges, only that it's currently not used there; a future implementation should be fine with a list as well.