add partial-search implementation

fixes #25.

fyi, it was much faster faster on a meager 300 nodes: 

via rest:

```
real	0m22.518s
user	0m0.000s
sys	0m0.020s
```

via partial search:

```
real	0m3.320s
user	0m0.008s
sys	0m0.008s
```